The Hmong fear persecution if they are sent back
Fears are growing for the safety of about 4,000 Hmong refugees, subject to deportation from Thailand within days.
The head of the United Nations refugee agency, Antonio Guterres, has urged Thailand to call off its plan to send the ethnic Hmong back to Laos.
The United States has expressed concern and Amnesty International said it was "appalled" by the deportation plan.
The Thai government says it will act according to the law, and a deal with Laos to send them back by 31 December.
In the past week, the army has sent dozens of large trucks to the camp and thousands of soldiers, according to reports in Thai media and phone interviews with residents in the area.
The UN's Mr Guterres said returning the refugees would not only endanger them but set a very grave example as, under international law, refugees could not be forcibly returned to countries that might persecute them.
The Hmong, being held at a camp in northern Phetchabun province, say they face persecution in Laos because they fought on the side of the Americans during the Vietnam War.
"Thailand has the responsibility and international obligation to ensure that any return of recognised refugees or other persons in needs of international protection... is undertaken on a strictly voluntary basis," Mr Guterres said.
The US has raised the issue many times with Bangkok, most recently this week during the visit of a senior State Department official.
Nine US senators sent a letter to Thai Prime Minister Abhisit Vejjajiva to express concern about the possible repatriation and criticise the government's screening process to determine refugee status, saying it was led by the military and lacked a civilian presence.
"Undoubtedly many of them have valid fears of persecution if they are returned to Laos," said Donna Guest, Amnesty's deputy Asia-Pacific director.
"We also know of people who have already been sent back who have been tortured or are missing, and moreover there has not been international access on a regular basis to these returnees, so that's a very big concern."
She told AFP news agency that Thailand "has completely ignored everybody's calls and we are appalled by this...".
"We will act according to the law, and we will be very careful," Mr Abhisit told reporters.
"We have measures to take care of this without human rights violations," he said.
Soon after he became prime minister a year ago, his image was damaged by revelations that the Thai army was beating and sending away boat-loads of ethnic Rohingya refugees, fleeing persecution in Burma.
Analysts have said such incidents show the prime minister's weakness relative to the priorities of the Thai military.