On Tuesday Andrej Mahecic, a UNHCR spokesman, said a solution for the detained Hmong "will not only respond to an urgent humanitarian need but also help turn one of the final pages in the refugee history of the Hmong in Thailand".
"They have not committed any crime and their detention serves no purpose," he told a news briefing in Geneva.
Known as America's "forgotten allies", the ethnic group from the remote mountains in Laos were recruited by the US Central Intelligence Agency to fight alongside US forces during the Vietnam War.
Over 300,000 Laotians, mostly Hmong, fled to Thailand after the Pathet Lao communists took power in 1975, citing political persecution.
Most were resettled in third countries.
Large numbers have since been resettled in western countries, including the US.
Several developed countries have offered to resettle the group of 158 Hmong who were rounded up in Bangkok in late 2006.
"Four countries – the United States, Australia, Canada and the Netherlands – have offered resettlement places to the refugees, and we believe they should be allowed to leave Thailand for resettlement," Mahecic said.
He told Reuters that the detained Hmong group, which includes women and babies born in detention, are "part of the legacy left by a troubled past".
"We don't want to see them forcibly returned."