The United Nations human rights commission has denounced the Australian government for withdrawing support to former Manus Island detainees as the stand-off over the provision of services to more than 600 men enters its fifth day.

It comes as detainees on Nauru call for an investigation into the death of a 29-year-old asylum seeker, who was killed in a road accident on the island nation on Thursday.

In a statement, Rupert Colville, spokesman for high commissioner Zeid Ra’ad Al Hussein, said the UN’s human rights committee had “serious concerns about the welfare, safety and wellbeing” of the men who remained at the centre in Papua New Guinea, which was formally decommissioned on Tuesday.

Water to the centre and essential medical services were also cut off on Tuesday, while electricity was disconnected on Wednesday morning. The men have been digging for water.

Colville said the men were “too frightened to leave”.

“We call on the Australian government, as the party who interned the men in the first place, to immediately provide protection, food, water and other basic services which have been cut off since authorities shuttered the facility on 31 October,” he said.

The commission repeated its criticism that Australia’s offshore refugee processing regime was “unsustainable, inhumane and contrary to its human rights obligations” and said asylum seekers and refugees should be immediately transferred to Australia for processing.

Rallies in support of the Manus refugees were held in Canberra and Brisbane on Friday and planned for Melbourne and Sydney on Saturday.

Colville said Australia and PNG had responsibilities under the 1951 Refugee Convention to protect and provide for the men.

“The men have said they fear they will be subjected to violence at the hands of locals if they leave the compound – and, given there have been violent incidents in the past, we believe these fears should be respected and satisfactorily addressed,” he said.

‘All migrants, including refugees and asylum-seekers, are human beings. Like all of us, they have a right to a safe and secure environment, a right to an adequate standard of living and to participate in the decision-making process that is affecting their future.

“Human dignity should be ensured throughout the entire processing of their asylum claims. We urge the governments of Australia and Papua New Guinea to fully respect their human rights, including their specific rights under international refugee law, and to enter into a dialogue with the men to ensure these rights are duly respected, protected and fulfilled.”

The Australian government has rejected claims it abandoned the detainees, saying water, power and medical services are available at three alternative accommodation facilities that can be “accessed at any time,” and that comfortable and secure transport to take them to those facilities is also “standing by”.

The government has not addressed a report made by the UN high commissioner for refugees, which inspected the alternative accommodation this week and said one of the units was not ready for occupation.

On Nauru, police reportedly arrested a man connected to a fatal crash that resulted in the death of a 29-year-old asylum seeker from Bangladesh, but detainees at the Nauru regional processing centre were concerned about the circumstances of the accident.

The man, who has been in Nauru for more than three years, died in a collision between the motorcycle he was riding and another vehicle on the road to Fly Camp, the detention centre for single men, about 2pm local time on Thursday.

Guardian Australia has attempted to contact the Nauruan government and the Australian immigration department to confirm the arrest.

Refugee Action Coalition spokesman Ian Rintoul has called for a full investigation to determine the circumstances of the crash.

“There is a lot of suspicion about what has happened,” Rintoul told Guardian Australia. “The fact that refugees are routinely hated just heightens their concerns even more.”

The government of Nauru released a statement on Thursday saying “reports on social media alleging this was more than an accident are false”.

“Police are investigating the accident and no other details are yet available,” the statement said. “The Government of Nauru extends its sincere sympathy to the family and friends of the deceased.”

On Friday, opposition leader Bill Shorten urged prime minister Malcolm Turnbull to accept a renewed offer from New Zealand to take 150 refugees from Australia’s offshore processing camps, saying it was similar to the US resettlement deal brokered with former US president Barack Obama.

Australia rejected the offer when it was first made by former New Zealand prime minister John Key in 2013, saying people smugglers would see it as a way to provide backdoor entry to Australia.

Turnbull is scheduled to meet New Zealand’s new prime minister, Jacinda Ardern, this weekend.