The United Nations has expressed "grave concern" over the treatment of two asylum seekers, who say they were savagely beaten by Papua New Guinea police and immigration officials on Manus Island before being arrested on New Year's Eve.

The UN's country team in PNG has urged the PNG government to investigate allegations of excessive use of force, torture and harassment of asylum seekers by officials, and to bring the relevant persons to justice if found guilty.

The intervention comes as officials from the United States' resettlement support centre prepare to visit PNG for their first round of interviews next month.

Refugees on Manus were told on Thursday that American immigration officers would come to the island February 14 and that the process would take between six and 12 months. They were also told there was no need for them to leave the detention centre in order to be eligible for resettlement.