Thailand expelled thousands of ethnic Hmong asylum-seekers from a refugee camp yesterday before their planned deportation to Laos, sparking angry criticism from the United States and Europe.

About 5,000 troops armed with batons and shields were sent to a mountain camp in Huay Nam Khao, 300 kilometres north of Bangkok to clear the 4,400 Hmong, who say they face oppression by Laos' communist government if sent back.

There was no violence, said Thai Colonel Thana Charuvat, who is in charge of the repatriation. Some 300 Hmong who had initially refused to leave the camp had agreed to end their resistance after several hours of negotiations, he said.

Reporters were barred from the camp during the operation and no independent confirmation was immediately available.

More than 4,000 had already been taken out of the camp in army trucks on the way to an immigration centre in Nong Khai bordering Laos, prior to being handed over to Lao authorities, the colonel told reporters.

Known as America's "forgotten allies," the Hmong sided with the United States during the Vietnam War and many fled Laos in 1975 when the communist Pathet Lao took power. Tens of thousands have since been resettled in the US.

The United States said the Thai operation was a "serious violation of the international humanitarian principles that Thailand has long been known for championing."

France said it "deplored" the Thai decision and UN High Commissioner for Refugees Antonio Guterres urged Bang-kok to stop the deportations.