Authorities in Papua New Guinea have demolished makeshift shelters and cut access to water storage at a decommissioned detention center on Manus Island, detainees said, piling pressure on about 600 asylum seekers who have until Saturday to clear out.

"Police and immigration destroyed our shelters," wrote Iranian asylum seeker Behrouz Boochani in a Tweet on Friday morning. "Inside the rooms is very hot without power for fans. We built these shelters to provide shade and cover from tropical sun and rain."

An hour later he added: "They destroyed the rubbish bins where we have been collecting water too."

Squalid conditions in the controversial refugee camp have been a source of shame for Australia, which ran the detention center, drawing sharp criticism from the United Nations and human rights groups. Last week the UN High Commissioner for Refugees urged Australia to act to "stop a humanitarian emergency unfolding."

Papua New Guinea authorities, who have said they would evict and "apprehend" those who do not leave immediately, put up a notice at the camp reading "force may be used" to relocate those remaining by Saturday.

About 20 men are thought to have left the camp, according three asylum seekers who spoke to Reuters, but the majority have chosen to remain.

The asylum seekers fear violence after threats from local residents.

Under Canberra's hardline immigration policy the Manus Island camp held asylum seekers who had sought to reach Australia by boat.

Australian private security shut down the center at the end of October after Papua New Guinea's Supreme Court ruled it unconstitutional, turning off taps and cutting electricity as they left.

About 600 asylum seekers refused to leave and set up makeshift water containers out of bins. Despite deteriorating conditions inside the camp, they fear reprisals from locals if they relocate to nearby transit centers.

The United Nations Human Rights Committee criticized Australia for allowing the Manus camp to close "without adequate arrangements for long-term viable relocation solutions for all refugees and asylum-seekers."

In a report published on Thursday the UN also called for Australia to stop rejecting refugees and bring its migration laws in line with international standards.

It expressed concerns about the Manus Island detention center including "inadequate mental health services, serious safety concerns and instances of assault, sexual abuse, self-harm and suspicious deaths."

Australia refuses to settle any refugees who try to arrive by boat. The government claims it dissuades asylum seekers from attempting the dangerous ocean crossing from Indonesia.

As a result Manus Island asylum seekers, along with those at a similar camp in Nauru, have no chance of reaching Australia.

Attempts at resettling in third countries have so far proved unsuccessful. A deal struck with the United States to take up to 1,250 refugees has seen only 54 accepted.

an/kms (AFP, AP, Reuters, dpa)