Under a Laos-Thai repatriation pact, the Thai government is to send nearly 8,000 Hmong back to Laos.
According to the Thai military supreme command's border affairs office, which manages the Petchabun camp, the 12 volunteered to go back to Laos as a goodwill gesture prior to Sundaravej's visit.
Aid group's concerns
But the UN refugee agency is concerned about the repatriation because of reports that they were sent back involuntarily, a spokeswoman said on Friday.
The Hmong have been living in Laos for more than 100 years
More than 200,000 Hmong have fled Laos since 1975
Thailand has initially taken in some Hmong into camps for resettlement or repatriation
About 90 per cent of Hmong refugees have been resettled in the US
Source: Migrationinformation.org; factfinding.org
A humanitarian aid group which works at the camp said one of the volunteers was a woman who had five children left behind at the camp.
Doctors Without Borders said the separation from her children suggested that her return was not voluntary.
The displacement of Hmong is a legacy of the Vietnam War.
During the 1960s and '70s, the US Central Intelligence Agency recruited Hmong fighters in Laos to attack neighbouring Vietnam's communist forces and their supply lines.
After the conflict ended in 1975, many who fought on the American side fled Laos, fearing persecution.
International monitors have been barred from the country but the Laos government says the Hmong will be safe.
In a recent incident, Hmong secretly filmed themselves locked up in the Nong Khai immigration detention centre.
Up to 153 people were filmed crammed in a temporary holding cell since December 2006. More than half were children.
They had staged hunger strikes, and some were threatening to kill themselves if they were not released soon.
Some 15 months ago, Thai immigration raided a poor suburb of the capital, Bangkok, along the Bang Sue Canal. They rounded up more than 150 Hmong.
At the time, most were carrying UNHCR refugee certificates.
The group was sent to prison despite having their UNHCR special status.
Several countries have agreed to take them, yet every day they wait to hear their fate.
Since then, hundreds of Hmong have fled the area.