The United Nations sounded an alarm yesterday over Australia's decision to stop providing services to migrants detained on the island of Nauru, saying the move would put vulnerable people at heightened risk.

Nauru is among the island nations involved in Canberra's policy of offshore processing for refugees and asylum seekers that try to reach Australia.

Australia had previously taken responsibility for providing basic care for migrants held on the island but has decided to let Nauru's authorities take over, with some financial support from Canberra.

The U.N. refugee agency's chief for Asia and the Pacific, Indrika Ratwatte, told reporters that he had "deep concern" over the transfer of services. He said it was vital that "adequate services are provided by trained, qualified individuals, particularly in mental health."

Ratwatte, who recently visited the island, reported that 80 percent of migrants detained there have been clinically diagnosed with post-traumatic stress disorder, trauma and depression.

While they are already lacking adequate care, Ratwatte said their plight could worsen if Nauru's authorities take charge without being properly supported.

Meanwhile, Nauru has also abolished links with Australia's highest court, which for four decades had been the final venue for migrants looking to appeal against adverse rulings on asylum claims.

Nauru has pledged to set up its own local court of appeals, but activists warn that in the interim asylum-seekers will be deprived of basic legal rights. Ratwatte said any legal gap was worrying. "Until such time that Nauru has established a process where appeal can be effectively heard, I think Australia should continue to have that facility open," he said.

More than 300 men, women and children are being held in a camp on Nauru, according to recent Australian immigration detention figures. The group includes nationals of multiple countries, including Afghanistan, Iran and Myanmar.