Twenty-two men from Manus Island have become the first refugees from Australia's offshore detention centre to leave Papua New Guinea for the United States.

Key points: US officials say refugees were approved under stringent vetting process

US officials say refugees were approved under stringent vetting process Refugee advocates say 27 others due to leave Nauru on September 27

Refugee advocates say 27 others due to leave Nauru on September 27 Refugees accepted under deal between Barack Obama and Malcolm Turnbull

The men flew out of Port Moresby this morning, bound for Manila, where they will be transferred to flights to the US.

The refugees were accepted under a deal between former US president Barack Obama and Prime Minister Malcolm Turnbull.

The Public Affairs officer for the US embassy in Port Moresby, Beverly Thacker, said they would be resettled in a number of different states.

"They have been approved for travel to the US under the same stringent vetting process that apply to all refugees," she said.

"They will be resettled under our normal refugee resettlement processes."

That means the men will be given housing for a set time and some cultural familiarisation.

Refugee advocates said another group of at least 27 people on Nauru, including women and children, were due to depart on September 27.

Many other refugees on both Manus Island and Nauru have been interviewed by US Homeland Security officials and are awaiting notice about their applications for resettlement.

The US is not bound to take a set number of refugees, so the final number of people in Australia's offshore detention centres who will be resettled remains unknown.

US President Donald Trump has previously expressed anger at the deal, but the White House has repeatedly confirmed it would honour its commitment.

The Australian and PNG Governments are pressing ahead with the closure of the Manus Island detention centre by October 31, although an estimated 500 to 600 men remain inside.