Canberra: Another 18 refugees from Australia’s offshore immigration regime have left Papua New Guinea for resettlement in America, but the United Nations refugee agency has warned that those who remain are showing “a pervasive and worsening sense of despair”.

Speaking in Geneva, the UNHCR’s regional protection officer, Rico Salcedo, said Australia’s offshore regime was holding refugees and asylum seekers in dangerous and harmful conditions, and alternative solutions must be found.

“We cannot emphasise enough that solutions must be found for all, outside of Papua New Guinea, as a matter of urgency,” Salcedo said. “Australia remains ultimately responsible, as the state from which these refugees and asylum seekers have sought international protection, for their welfare and long-term settlement outside of Papua New Guinea.”

The UNHCR has maintained an increased presence on Manus Island since the standoff last year over the closure of the former detention centre and the refugees’ forced eviction from the camp. The men have since been moved to three other Australian-built and controlled centres nearer to Lorengau township, but which have poorer facilities and fewer services. Health services, in particular care for those with chronic conditions and mental health issues, are especially limited.

The Australian government has already conceded to pay $70 million to nearly 2,000 refugees and asylum seekers it held on Manus.

“What stood out the most from this mission at the time we were there was a pervasive and worsening sense of despair among refugees and asylum seekers,” Salcedo said. “In our conversations with different people, there’s a sense of desolation. People are grasping for hope.

“I spoke with a refugee who shared with me his daily struggles and what he was going through. He told me how he was concerned about some of his friends who are suffering [from] depression, who were thinking of self-harm and how he tries to be there for them. He also shared how he felt unable to help on some days because he himself could not get the help he needed.”

The US deal, which has now resettled 85 people from Manus Island in America, was “an ongoing and welcome process” but that “the knowledge that many remain without any resolution is weighing on everyone”.

Salcedo said the government of Australia should assume a clear coordination role over the services it has contracted private companies to provide.

“Clearly, much more needs to be done to bring the circumstances of refugees and asylum seekers on Manus Island up to a basic minimum standard. These critical steps, however, will only be a stopgap measure until durable solutions are found and made available for them outside of Papua New Guinea.”

On Wednesday, the home affairs minister, Peter Dutton, told parliament that Australia would not be changing its asylum policies or undoing the controversial offshore processing regime.

“If you undo the policies, if you allow people to come from Manus and Nauru ... you will restart the boats. That is very clear. It’s very clear on the intelligence available to us that if you close down regional processing centres, that if you don’t turn back boats where it’s safe to do so, and have the resolve that we’ve shown, the boats will recommence.”

The latest cohort of 18 refugees — Afghan, Pakistani and Myanmar Rohingyan men — left PNG on Tuesday. It is only the third cohort to leave Manus Island in the 15 months since the US deal was announced.

About 50 refugees have been resettled in that time from Nauru, but the Australian government has conceded the deal with the US government — with a publicly declared cap of 1,250 places in America — will not be enough to resettle all of those it holds offshore.

PNG has offered to resettle some refugees, but is demanding greater assistance from Australia to do it. Fewer than 30 refugees have been able to settle in PNG outside of Australia’s offshore immigration control.

Nauru has resolutely refused to permanently resettle any refugees.