US officials interviewing refugees held in an Australian-run offshore detention centre have left the facility abruptly, throwing further doubt over a plan to resettle many of the detainees in America.

US officials halted screening interviews and departed the Pacific island of Nauru on Friday, two weeks short of their scheduled timetable and a day after Washington said the US had reached its annual refugee intake cap.

"US (officials) were scheduled to be on Nauru until July 26 but they left on Friday," one refugee told Reuters, requesting anonymity as he did not want to jeopardise his application for US resettlement.

In the US, a senior member of the union that represents refugee officers at US Citizenship and Immigration Services (USCIS), a Department of Homeland Security agency, told Reuters his own trip to Nauru was not going forward as scheduled.

Jason Marks, chief steward of the American Federation of Government Employees Local 1924, told Reuters his trip has now been pushed back and it was unclear whether it will actually happen.

The USCIS said that the program would continue but offered no details.

"We do not discuss the exact dates of USCIS' circuit rides to adjudicate refugees' applications. However, we are planning return trips," the agency said in a statement.

"It is not uncommon for the dates of tentatively-planned refugee circuit ride trips worldwide to change due to a wide variety of factors."

The Australian Immigration Department declined to comment on the whereabouts of the US officials or the future of a refugee swap agreement between Australia and the US that President Donald Trump earlier this year branded a "dumb deal".

An indefinite postponement of the deal would have significant repercussions for Australia's pledge to close a second detention centre on Papua New Guinea's Manus island on October 31.

Only 70 refugees, less than 10 per cent of the total detainees held in the camp, have completed US processing.

"The US deal looks more and more doubtful," Ian Rintoul from the Refugee Action Coalition said. "The US deal was never the solution the Australian government pretended it to be."

Former US President Barack Obama agreed a deal with Australia late last year to offer refuge to up to 1250 asylum seekers, a deal the Trump administration said it would only honour to maintain a strong relationship with Australia and then only on condition that refugees satisfied strict checks.

In exchange, Australia has pledged to take Central American refugees from a centre in Costa Rica, where the US has taken in a larger number of people in recent years.

The swap is designed, in part, to help Australia close both Manus and Nauru, which are expensive to run and have been widely criticised by the United Nations and others over treatment of detainees.