Concerns are mounting for a group of at least 26 Hmong children who have been separated from their families in Thailand and are now in Laos.
The children disappeared from a camp in Phetchabun, Thailand
According to the UN's refugee agency, the children are believed to be in the town of Paksan, east of Vientiane, but it is unclear how they got there.
The children had been living in a camp for refugees in north-eastern Thailand.
Their families arrived in Thailand last summer, claiming they were being persecuted by the Lao government.
HMONG IN LAOS
Ethnic group that often complains of marginalisation in Lao society
Took the side of the US in the Vietnam War - and say they are persecuted because of it
Many still live in jungles
Small numbers say they are fighting rebel insurgency
Thousands have fled to Thailand in recent years
US took in 14,000 Hmong recently, but has no plans for taking more
Concern had already been voiced for these families, who said they undertook the arduous journey across the border because they mistakenly believed the US was still taking in Hmong from Thai refugee camps.
They have yet to be given legal permission to stay in Thailand, and are currently living in a temporary camp in Phetchabun province, where they are being helped by aid agencies.
Since discovering the children - many of whom are teenagers - have disappeared, international organisations have been trying to facilitate their return.
"There are now discussions going on between the Thai and the Lao governments to try to reunite them with their parents," said UNHCR spokesman Bhairaja Panday.
"It's an issue that everyone is concerned with," he added.
It is still not clear how the children ended up in Laos, although there is speculation that Thai officials secretly deported them back over the border when they wandered outside the temporary area designated for them by the Thai authorities.
"There are indications the children were found outside the camp," said Mr Panday.
On Thursday Laotian foreign ministry spokesman Yong Chanthalangsy told the French news agency AFP: "The US embassy informed us that 26 children have been forced by Thai police to cross the border."
"These activities of the Thai police are illegal and are in violation of the existing mechanisms between the two countries," Mr Yong said.
Analysts say the issue highlights the need to clarify the fate of the 5,000 Hmong refugees still living in Phetchabun.
Last July, Thailand threatened to repatriate them en masse, but the UNHCR cautioned against any mass deportation, asking instead that each case be assessed individually.