"Four years on, more than 2,000 people are still languishing in unacceptable circumstances.
Families have been separated and many have suffered physical and psychological harm," UNHCR chief Filippo Grandi said in a statement.
According to the statement, around 2,500 refugees and asylum-seekers have been forcibly transferred by Australia for offshore processing of their applications in Papua New Guinea and Nauru since the introduction of the current policy in 2013.
Of these, some 1,100 remain in Nauru and 900 in Papua New Guinea, added the statement.
The statement adds that the Australian government informed the UNHCR it will not accept refugees on its territory and offered the options of either letting them be where they are, or transferring them to Cambodia, with whom it signed an agreement in 2014, or the US, with whom it signed an agreement in 2016, to transfer some of the refugees detained in Papua New Guinea and Nauru.
"To avoid prolonging their ordeal, UNHCR has no other choice but to endorse the relocation of all refugees on Papua New Guinea and Nauru to the US, even those with close family members in Australia," the UN High Commissioner said in his statement.
Many of the refugees in these offshore centres have fled strife-torn countries such as Afghanistan, Darfur, Pakistan, Somalia and Syria, and others have escaped discrimination or status as stateless persons such as the Rohingya minorities from Myanmar.