SYDNEY, Nov 18 (Reuters) - Australia is responsible for children detained for years in an offshore immigration center developing post-traumatic stress disorder and other ailments, a United Nations investigator into the human rights of migrants said on Friday.

The findings of U.N. Special Rapporteur Francois Crépeau are among the most critical to date by the international agency of Australia’s controversial detention policies. They come amid a global debate on how to manage huge numbers of asylum seekers displaced by conflict.

“These children show signs of [post-traumatic stress disorder], anxiety, depression and have symptoms such as insomnia, nightmares and bedwetting. This is not acceptable,” said Crépeau.

Crépeau, one of just a handful of international observers to go to the detention center on the tiny South Pacific island of Nauru, said children were bearing the brunt of Australia’s harsh offshore detention policy.

PETER PARKS via Getty Images An activist in Sydney holds a placard in an event organized by Doctors for Refugees to demand humane treatment of asylum seekers and refugees.

Under Australian law, asylum seekers intercepted trying to reach the country by boat are sent for processing at camps on Nauru and on Manus Island in Papua New Guinea. Almost 1,200 people are being held indefinitely in the camps.

Australia reintroduced offshore detention in 2013 after a spate of maritime arrivals and drownings at sea. Roundly criticized, its harsh immigration policy has bipartisan support and is an important issue among Australian voters.

Australia’s Department of Immigration said in a statement it “does not accept some of the preliminary observations” made by Crépeau and that its policy complied with “international obligations and human rights principles.”

Crépeau also rejected Australia’s claim that Nauru and PNG were responsible for the offshore processing centers.

The criticism of Australia’s policy of indefinite detention of would-be asylum seekers came days after the United States agreed to take a “substantial” number of those held on Manus Island and Nauru.

That followed Australia agreeing to join a U.S plan to resettle Central American refugees being held in Costa Rica.

The deals raise the prospect that those seeking asylum in Australia could be settled in the United States, while those in Costa Rica could be settled in Australia.

The U.S.-Australia deal may yet be scuppered because it will require approval from U.S. President-elect Trump.

Crépeau also criticized Australia’s policy of mandatory detention for those who arrive in Australia without necessary visas, which he said violated international law.

“Many of them are on a regular diet of happy pills in the morning and sleeping tablets in the evening,” he said.