The Australian government has announced a landmark “one-off” resettlement deal to the United States for some refugees held at Australia’s remote offshore detention facilities on Nauru and Manus Island.

On Sunday the prime minister, Malcolm Turnbull, and the immigration minister, Peter Dutton, announced a deal that would prioritise families, women and children for settlement with “the prospect” that some others of the 1,616 people found to be refugees so far in offshore detention would be resettled in the US.

“I can now confirm that the government has now reached a further third-party resettlement arrangement,” Turnbull said. “The agreement is with the United States. It is a one-off agreement. It will not be repeated. It is only available to those currently in the regional processing centres.”

“It will not be available to any persons seeking to reach Australia in the future. Our priority is the resettlement of women, children and families.”

At the press conference, at Maritime Border Command in Canberra, Turnbull refused to say how many refugees the deal would apply to but said the deal showed the government “can source and provide alternative resettlement options”.

The remaining refugees on Nauru will be eligible for 20-year temporary visas on Nauru.

The US resettlement would be contingent on vetting by the United States Homeland Security agency.

Turnbull said that the scheme would be “administered with” the United Nations High Commissioner for Refugees.

Speaking in New Zealand before the announcement, US secretary of state John Kerry said the US had agreed to “consider referrals from [the] UNHCR on refugees now residing in Nauru and Papua New Guinea”.

But a statement from the UNHCR said while it welcomed the deal and would endorse referrals to the US, it was “not a party to it” and had no formal role in processing.

“The arrangement reflects a much-needed, long-term solution for some refugees who have been held in Nauru and Papua New Guinea for over three years and who remain in a precarious situation,” the statement said.

“It is on this basis that UNHCR will endorse referrals made from Australia to the United States, on a one-off, good offices, humanitarian basis, in light of the acute humanitarian situation. The full details of the agreement are not yet known, and UNHCR is not a party to it.”

Australia’s opposition leader, Bill Shorten, also offered his cautious support for the resettlement deal.

“It has taken the government three-plus years to negotiate this deal, but we are pleased if it is an end to indefinite detention,” Shorten said.

“We will certainly in principle work with the government. But we do welcome this. We do want to see people moved out of these facilities.”

The Greens’ immigration spokesman, Nick McKim, welcomed the deal as an admission indefinite offshore detention was unacceptable but added there were “still significant levels of uncertainty” for refugees because the number eligible and the timeframe is unspecified.

The Human Rights Law Centre’s director of legal advocacy said the announcement was important but “long overdue”.

“Today’s announcement may be a starting point, but there is an awfully long way to go,” he said.

Human Rights Watch’s Australia director Elaine Pearson said the deal was not perfect but was a “major step forward” to resolving the impasse of Australia’s asylum seekers.

“We look forward to seeing more details about this agreement, and we urge both the US and Australia to move quickly to avoid more mental anguish and trauma for people who have suffered so much,” she said.

If the government’s resettlement plan succeeds it is likely to reduce the populations of Australia’s notorious regional offshore detention centres.

Over the last three years the remote facilities have been plagued by reports of shocking conditions, poor management and deteriorating mental health of asylum seekers.

The Guardian’s publication of the Nauru filesshowed the devastating trauma and abuse inflicted on children on Nauru.

An asylum seeker was beaten to death by guards in a wave of unrest on Manus Island in February 2014, and on Nauru in May 2016 an asylum seeker self-immolated in front of staff from the UN high commissioner for refugees.

Asked about the potential closure of the Manus Island detention facility in the wake of the announcement, Dutton noted Papua New Guinea had announced it would close and he had nothing to add.

But he said that “no element” of the government’s policy on offshore detention would change, and Australia would “still rely on regional processing which is why Nauru will remain in its current status forever”.

According to the latest figures from Australia’s immigration department there are 872 people held in the Manus Island detention centre and 390 held in the Nauru facility.

A number of refugees are also currently on the mainland in Australia undergoing medical treatment for serious physical or mental conditions who may be eligible for the resettlement deal.

The Australian government’s deal will only apply to those who have received positive refugee determinations on Manus Island and Nauru. On Manus Island there have been 675 positive refugee determinations out of 1,015 and on Nauru there have been 941 positive determinations out of 1,195.

Turnbull said he anticipated people smugglers would “use this agreement as a marketing opportunity” but the government had “put in place the largest and most capable maritime surveillance and response fleet Australia has ever deployed” to stop journeys by sea.

“Any people smuggling boats that attempt to reach Australia will be intercepted and turned back.”

Speculation the Australian government was preparing to finalise a resettlement deal with the US was sparked by the announcement in September it would take refugees from camps in Costa Rica.

It ramped up two weeks ago when the government proposed a lifetime travel ban on resettled refugees visiting Australia, a measure Dutton said needed to be in place to facilitate third-party resettlement.

Labor has opposed the measure, labelling aspects of the plan “ridiculous” because it would prevent a refugee resettled in a third country from coming to Australia as a tourist or on a business trip.

Turnbull ramped up pressure on Labor, accusing it of opposing the ban due to “theoretical possibilities” that refugees would want to visit Australia as tourists in 30 or 40 years.

The opposition should instead focus on “the security of Australia’s borders today and tomorrow”, the prime minister said, describing the travel ban as important to send the “strongest and most unequivocal message to people smugglers” that no refugees would reach Australia.

Among those on Manus and Nauru are people from Iran, Syria, Sri Lanka, Pakistan, Bangladesh and Iraq, as well as some who are stateless.

Behrouz Boochani, an Iranian journalist detained on Manus Island, told Guardian Australia refugees he had spoken to would be happy to go to the US.

“Be sure that most people would love to go to America, but some people need to join their families in Australia,” he said. “I think Australia has to respect them because they want to join their families.”

The Australian government has announced a new resettlement deal for refugees held on Manus Island and Nauru.

The prime minister, Malcolm Turnbull, said on Monday that some refugees would now be eligible for resettlement in the United States as part of a “one-off” deal.

Some details about how the resettlement arrangements work have been released but much remains unclear. Here’s what we know so far.

Where will refugees on Manus Island and Nauru be able to be resettled?

The announcement on Sunday adds the United States to a short list of resettlement options for refugees on Manus Island and Nauru.

The deal will apply to people who are currently held in the offshore detention facilities and who are assessed as refugees by the Papua New Guinea foreign minister and the Nauruan government. It will also apply to any of the 370 refugees now in Australia receiving medical care if they have been found to be refugees.

If a refugee misses out on US resettlement, the existing options of Papua New Guinea and Cambodia are still in place, although only a handful have been resettled in Cambodia.

In the case of Papua New Guinea, as recently as October only 24 had settled there. Many were afraid for their safety and some attempted to return to Manus Island’s detention centre.

The immigration minister, Peter Dutton, revealed on Sunday the government was in the final stages of negotiating a 20-year visa with Nauru. This is an extension to the initial commitment of five year temporary visas that Scott Morrison made in 2014.

How many people will be resettled in the United States?

The government has not outlined how many people will be eligible for resettlement or how it will determine who can apply. When asked about the numbers, Turnbull would only say that the deal prioritises families, women and children who are held on Nauru. He said United States Homeland Security officials would be starting a vetting process in coming days.

We do know that as of October 2016 there was a total of 1,616 people on both Manus Island and Nauru who had positive refugee determinations. This is the highest number of people who could be eligible for resettlement at this point in time.

There are also still a number of people at both facilities who have not yet had their claims processed or are undergoing appeals, so the final numbers eligible for resettlement remain unclear.

There are 390 people held in what is known as the Nauru “open centre” and 872 in the Manus Island facility. Generally these groups have not had their claims processed or have received negative refugee assessments.

When pressed on the idea that many refugees may never get the option to go to the US, Turnbull said the government had “demonstrated we can source and provide alternative resettlement options”, hinting at the possibility of further deals but falling short of saying the US deal would be sufficient to empty detention facilities.

What will the Trump factor mean?

The election of Donald Trump could create some difficulties for the United States deal. Bilateral arrangements like these are made by the executive and can be altered or revoked depending on the government of the day.

Faced with several questions about whether Trump could spell trouble for the deal Turnbull said: “We deal with one administration at the time. There is only one president of the United States at a time.”

“These arrangements have had a long run-up. The agreement was reached some time ago,” he said.

Asked about Trump’s policy to ban Muslim immigration, Turnbull stressed Australia’s history of cooperation with the US on humanitarian goals but did not say whether he thought the deal could survive a Trump presidency.

What will happen to asylum seekers not found to be refugees?

The US deal does not apply to those not found to be refugees.

People who are not granted refugee status should return to their home country, Malcolm Turnbull said on Sunday.

Australia has encouraged asylum seekers to return to their countries of origin with payments. Involuntary returns have not yet occured on Manus Island or Nauru, but there have been indications from Papua New Guinea that this may be a future possibility.

For the moment though those who decline to return currently face the prospect of indefinite detention.

When will resettlement occur?

The prime minister said the process would be “orderly” but the government would not set a definite timeline for the US resettlement deal.

“There will be American officials from Homeland Security coming to Australia to begin the process in the next few days,” he said.

“This will be an orderly transition and, of course, the American government will carefully assess each would-be transferee to the United States.”

What role with the United Nations high commissioner for refugees play?

Turnbull said that the scheme would be “administered with” the UNHCR. This comment is put considerably higher than what the refugee agency itself has stated.

The UNHCR said while it welcomed the deal and would endorse referrals to the US, it was “not a party to it” and had no formal role in processing. 

This referral is an important part of the process, but it is in no way a processing role. Under US migration law a referral is needed to meet the criteria for eligibility set out under the United States humanitarian intake program.