Antonio Guterres, U.N. high commissioner for refugees, said refugees and asylum-seekers cannot be forcibly returned to countries or areas where they could face persecution.
Thailand has said it plans to deport the Hmong before the end of the month, in line with a bilateral agreement with Laos. The United Nations said at least 150 of the Hmong are recognized refugees.
"In accordance with international law, Thailand has the responsibility and international obligation to ensure that any return of recognized refugees or other persons in needs of international protection to their country of origin is undertaken on a strictly voluntary basis," Guterres said Thursday in a statement. "To proceed otherwise would not only endanger the protection of the refugees but set a very grave international example."
The United Nations said many Hmong living in the highlands of Laos participated in the military conflict that engulfed Laos in the 1960s and 1970s. Tens of thousands of Hmong fled to Thailand in search of asylum when Pathet Lao came to power in 1975.