Concern has been mounting over the fate of two boats, one reportedly carrying 153 Tamil Sri Lankan asylum-seekers and another with 50 on board, intercepted in recent days by the Australian navy in Australian waters.
Under its policy of not commenting on "operational matters", Canberra has refused to confirm the boats exist, sparking criticism from the media, rights advocates and the UN refugee agency UNHCR.
"UNHCR has followed with profound concern recent reports in the media and from the community in relation to the interception at sea of individuals who may be seeking Australia's protection," the agency said in a statement.
"Anything short of such a screening, referral and assessment may risk putting already vulnerable individuals at grave risk of danger.
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"International law prescribes that no individual can be returned involuntarily to a country in which he or she has a well-founded fear of persecution," the UNHCR said.
It came as The Australian newspaper said a mid-ocean transfer of some would-be refugees to a Sri Lankan naval vessel was imminent, with the government keen to maintain its record of no boatpeople making it to Australia for more than six months.
In Sri Lanka, while the navy maintained publicly that no transfer had taken place, an official source told AFP that at least 50 of its nationals could be returned.
"There are discussions underway and about 45 to 50 people who are supposed to have set off from the (Sri Lankan) east coast could be brought back by the navy in the coming days," the source said, asking not to be named.
Sri Lanka navy spokesman Kosala Warnakulasooriya insisted that no naval craft had been deployed to return its citizens.
Separately, the Sydney Morning Herald reported that those on board the boats were being asked just four basic questions via video link to the Australian boat that picked them up in assessing their claim for asylum.