(CNN) -- Thailand deported about 450 Hmong refugees to Laos Monday and plans on evicting 3,900 more from a refugee camp, the government said.
The forced repatriation prompted concern from human rights groups who say the refugees risk persecution in Laos.
Many Hmong sided with the United States during the Vietnam War when the conflict spread to Laos.
When the Communists assumed power in 1975, thousands of Hmong fled to neighboring Thailand.
The U.S. State Department said Monday that the Thai government had determined that many of the Hmong were in need of protection, and that such returns would "imperil the well-being of many individuals."
The Thai government says the Hmong at the Huay Nam Khao Camp in Petchabun province in the north had left Laos not for fear of persecution but for financial reasons. They entered Thailand illegally and could not claim political asylum, the government says.
On Monday, the Thai army put 448 Hmong on buses that were headed for Laos.
Many refused to leave peacefully, Army Col. Thana Jaruwat said on national television. If they did not comply, the army will have to "enforce the law," he said without elaborating.