The UN refugee agency said on Saturday at least six people registered with it were among 1,200 Myanmar nationals to be deported by Malaysia next week, while the United States voiced alarm that the plan could put deportees' lives at risk.
The comments came as Myanmar naval ships arrived in Malaysian waters to pick up the detainees.
Malaysia will deport the Myanmar citizens - including asylum seekers - after Myanmar's military, which seized power in a Feb. 1 coup, offered to send navy ships to pick them up, officials and refugee groups say.
Malaysia has vowed not to deport Rohingya Muslims or refugees registered with the UN High Commissioner for Refugees (UNHCR).
But concerns over deportation of unregistered asylum-seekers persist as UNHCR has not been allowed to interview detainees for more than a year to verify their status.
Malaysia does not formally recognize refugees and arrests them along with other undocumented migrants.
Confirming the planned deportation of six persons of concern registered with it, the UNHCR said it has asked authorities not to send back those in need of international protection.
"We are concerned that there may be others of concern to UNHCR in the group," Yante Ismail, a spokeswoman for the agency, told Reuters in an emailed statement.
Rights groups have asked Malaysia to scrap the deportation, saying it would endanger the deportees. Some of the deportees include people from Myanmar Muslim and Chin communities who arrived in Malaysia fleeing conflict and persecution at home.
The US embassy confirmed to Reuters that it has raised concerns and echoed calls for UNHCR access to those in immigration detention.
Malaysia's foreign ministry had no immediate comment.
The Myanmar Embassy in Malaysia did not respond to calls seeking comment. On Facebook on Saturday it confirmed it would be bringing back the 1,200 people, saying it was prioritising the repatriation of nationals stranded due to the pandemic.
Three Myanmar-flagged vessels were anchored off Malaysia's Lumut naval base on Saturday, including one described as a military operations ship, according to ship-tracking website Marine Traffic.
Two Malaysian sources, who requested anonymity, confirmed those ships were sent to pick up the detainees. They are scheduled to leave for Myanmar on Tuesday, Malaysia has said.
US and other Western missions in Kuala Lumpur have been trying to dissuade Malaysia from proceeding, four other sources with knowledge of the matter said.
"Malaysia is legitimising the military government by handing over the detainees," one source said.
Diplomats are also urging Malaysia to let the UNHCR interview the deportees and have expressed concern over Malaysia's cooperation with the Myanmar junta, the sources said.
Malaysia had earlier expressed "serious concern" over the coup.
Coup opponents have been protesting for weeks. Two people were killed on Saturday when police fired to disperse protesters.