The expulsion of the Hmong sparked international criticism
The UN refugee agency has formally asked Laos for access to more than 4,000 ethnic Hmong who were forcibly repatriated from Thailand.
The agency also called on the Thai government to provide details of a Thai-Laotian accord regarding the treatment of the Hmong.
The Hmong say they face persecution in communist-led Laos because they backed US forces during the Vietnam war.
But the Thai government regards the Hmong as illegal economic migrants.
In a statement from Geneva, the United Nations High Commissioner for Refugees (UNHCR) said it believed that some of the Hmong sent back to Laos had refugee status and needed "international protection".
THE FORGOTTEN HMONG
Ethnic group, mostly from the highlands of Laos
Many fought alongside US forces in the Vietnam War during the 1960s and 1970s, as part of a secret CIA-trained militia
When the Communists took power in Laos in 1975, many feared punishment for siding with the Americans
Some formed a rebel group inside the country that has now been largely quashed
More than 150,000 are known to have fled to Thailand, many of whom were housed in aid camps
Tens of thousands have since been resettled in the US
The statement urged Thailand to provide details of a Thai-Laotian agreement that governs how Hmong returnees are treated.
The agency, which has no formal presence in Laos, asked the Thai government to ensure that any commitments made under the accord were "honoured".
Over a period of 24 hours, beginning on Monday, more than 4,000 people were removed from a camp in Thailand's northern Phetchabun province, where they had been living for the past five years.
UN refugee agency staff were not allowed to visit them beforehand.
A second group of 158 Hmong, who were officially recognised as refugees by the UNHCR, were also forcibly repatriated, after three years in a detention centre.
The removal of the Hmong, who are sometimes known as America's forgotten allies, has sparked international criticism.
Ariane Rummery, a spokeswoman for the UNHCR in Bangkok, told the BBC that offers had been made by third countries to resettle those granted refugee status, but Thailand had consistently rejected the idea.
The Thai authorities say they have been given assurances by the government in Laos that the returnees will not be mistreated.
"Laos has promised Thailand that they will give good treatment to these people," Thai foreign minister Kasit Piromya told reporters on Wednesday.
"They will not be jailed and they will be given passports and a chance to meet with third countries that could resettle them."