Soldiers have ordered about 1,500 ethnic Hmong people to leave a refugee camp in central Thailand.
Many of those rounded up have relatives and friends in the camp
The Thai authorities said they had entered the area illegally, hoping to join a US resettlement programme.
About 14,300 Hmong people are legally living at the Tham Krabok camp, built around a Buddhist temple, after registering for resettlement in 2003.
But in recent months, many others have arrived in the camp, hoping they too will be allowed a new life in the US.
Up to 300,000 Hmong fled to Thailand from Laos, after the country fell under communist rule in 1975.
The Hmong fought for the US army during the Vietnam War and have feared reprisals ever since.
The resettlement programme was set up after the US agreed to accept a certain quota of Hmong refugees - all of them from the Tham Krabok camp.
'Everyone just wants to go back'
Hundreds of troops arrived at the camp in Saraburi province at around 0200 local time on Wednesday (1900 GMT Tuesday).
They rounded up any refugees who had failed to register before August 2003, according to Yeh Goo, a camp leader.
It is not immediately clear what will happen to those who have been barred from the camp.
"We don't know what's happening and we're waiting now,"
said Pao Thao, one of the refugees targeted by the soldiers.
"Everyone here just wants to go back
to the camp so we can go to America," he said.
General Pallop Pinmanee, one of the senior officers in
charge of the resettlement scheme, told Reuters news agency that the camp's population included about 2,000 unregistered Hmong.
"These people have failed to register with Thai authorities
for the repatriation scheme, but they want to sneak along with
those who have registered," he said.
"They live around the temple and our policy is clear, to
get the immigration police to arrest them as illegal
Meanwhile the resettlement programme is gathering pace.
Some 1,100 refugees have already left on commercial flights, and a further 289 people left on Monday on a specially charted plane.
People at the camp are anxiously waiting for their flights to the US
A second chartered flight has been organised at the end of August, and three more in September.
The US agreement to accept qualifying refugees from Tham Krabok camp was designed to answer long-running criticism that it had turned its back on the Hmong.
The Hmong's ties to the US are complicated. During the Vietnam War, large numbers of ethnic Hmong sided with the US army as the conflict spread into neighbouring Laos, and provided an important support role.
But at the end of the war, the US Government stopped its support for the Hmong.
When a communist movement ousted the US-backed Lao royal family in 1975, as many as a third of the Hmong population are thought to have fled.