SYDNEY, Australia — The United Nations High Commissioner for Refugees has accused the Australian government of reneging on a “clear understanding” that some refugees detained offshore who have close family ties in Australia could be resettled in the country.

In a statement on Monday, the commissioner, Filippo Grandi, said the United Nations agency had agreed in November to help with the relocation to the United States of refugees in detention centers on Manus Island in Papua New Guinea and on the island nation of Nauru after Washington agreed to take in hundreds of refugees being held by Australia at those centers. But the agency did so, Mr. Grandi said, on the condition that some refugees would be resettled in 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,” Mr. Grandi said.

But the commissioner said the agency had recently been told by the Australian government that it would not be accepting those refugees and that they, and others on Nauru and Manus, would have to remain at the detention centers or be transferred to Cambodia or to the United States.