The UN accused Australia Monday of backtracking on a deal to relax its strong stance on asylum-seekers and resettle some refugees now held in overseas detention on home soil.

The UN High Commissioner for Refugees (UNHCR) said it had agreed last November to help relocate the boatpeople to the United States on the basis that Canberra would accept some of them who have links to Australia.

"We agreed to do so on the clear understanding that vulnerable refugees with close family ties in Australia would ultimately be allowed to settle there," commissioner Filippo Grandi said in a statement.

"UNHCR has recently been informed by Australia that it refuses to accept even these refugees."

He added that they, along with the others in camps on the neighbouring nations of Nauru and Papua New Guinea, had been told their only option was to stay where they were or be transferred to Cambodia or the United States.

Australia sends anyone who tries to enter by boat without a visa to remote detention facilities on Nauru and Papua New Guinea's Manus Island.Even those subsequently found to be genuine refugees are barred from settling in Australia.

The immigration department said Monday this had long been the case.

"The position of the coalition government has been clear and consistent: those transferred to regional processing centres will never settle in Australia," said a spokesperson.

Immigration Minister Peter Dutton reinforced this on Sunday.

"No, people will not be coming to Australia. I have said that consistently, the prime minister has said it consistently, as did prime minister (Tony) Abbott at the time," he told Sky News.

Prime Minister Malcolm Turnbull struck a pact with former US president Barack Obama to resettle some refugees in the camps in America.

President Donald Trump has grudgingly agreed to honour what he has called a "dumb deal", although it remains unclear how many Washington will accept.

More than 1,000 remain offshore and Canberra's stance means "some with serious medical conditions, or who have undergone traumatic experiences, including sexual violence, cannot receive the support of their close family members residing in Australia", the UN said.

"To avoid prolonging their ordeal, UNHCR has no other choice but to endorse the relocation of all refugees on Papua New Guinea and Nauru to the United States, even those with close family members in Australia," Grandi added.

Rights groups, who have long accused the government of failing to uphold its international obligations, called on Canberra to honour the deal it had allegedly struck with the UN.

"The right thing to do, the humane thing to do, would be to immediately bring those trapped in Nauru and Manus Island to Australia to be reunited with their family members here,” said Amnesty International refugee coordinator Graham Thom.

Source: AFP