The Prime Minister has also placed a longer timeline on the closure of the Manus Island detention centre, saying he expects it to close by the end of this year. Previously, Papua New Guinean officials have spoken of closure by October to comply with a court ruling last April that asylum seekers were being held illegally.

Refusing to be drawn on the fate of those who are not resettled in America, Mr Turnbull congratulated Papua New Guinea for making "significant progress" in its attempts to resettle around 1000 asylum seekers who are in their fourth year in PNG.

But neither Mr Turnbull nor PNG Prime Minister Peter O'Neill could provide a number for those who have been successfully resettled, which is believed to be fewer than 15.

"It's clearly worked in a number of cases and the Prime Minister and I talked about that earlier," Mr Turnbull said, inviting Mr O'Neill to expand.

"We cannot force people in a resettlement exercise when they are not able to accept our offer," Mr O'Neill replied, after noting that many local businesses had put their hands up to employ refugees.

The two leaders addressed the media after an hour of talks in Port Moresby on Saturday, with Mr Turnbull thanking Mr O'Neill for his help in stopping "the scourge of people smuggling".

Around 300 of the refugees on Manus Island have been fingerprinted and photographed in the second phase of "extreme vetting" for potential resettlement in the US, under a deal negotiated with the Obama administration that President Trump has reluctantly agreed to honour. Another 300 are likely to be interviewed in coming months.

But there is no certainty about the process or timeline for their departure and, even if 600 are taken, that could still leave around 400 men who the United Nations refugee agency says have no possibility of being integrated into PNG.

"While the government and many people of Papua New Guinea have made commendable efforts to support settlement, it is now abundantly clear that even temporary integration for refugees in Papua New Guinea is out of the question," the UNHCR's regional representative in Canberra, Thomas Albrect, has told Fairfax Media.

A study late last year by three mental health experts for the UNHCR found rates of depressive or anxiety disorders and post-traumatic stress disorder among the asylum seekers and refugees on Manus "amongst the highest recorded rates of any surveyed population".

Mr Turnbull spoke of working with third countries on durable solutions for those now on Manus Island and Nauru who do not go to the US, but this is at odds with recent testimony to a Senate committee by immigration department head, Michael Pezzullo. Mr Pezzullo said there were no other third country agreements currently being negotiated.

Pressed on whether asylum seekers and refugees who are not included in the US agreement could be sent to Nauru, Mr Turnbull said: "We'll take this process on step at a time."