BANGKOK — Thailand on Monday began to deport back to Laos more than 4,000 ethnic Hmong asylum-seekers, defying intense pressure from the United Nations, the United States and human rights groups who say the deportees could face persecution upon their return.

After days of preparation, 5,000 troops and officials entered the Hmong camp in Thailand’s central Petchabun province early in the morning to begin moving the asylum-seekers onto buses that would take them over the border, a process that a military official said might take 24 hours.

Col. Thana Charuwat, the officer in charge of the operation, said that 2,100 of the camp residents had agreed to leave voluntarily and that the army was trying to persuade the rest. But the Thai government has blocked media and international access to the camp and mobile telephone signals in it, making it difficult to independently confirm that information.

The migrants say they are at risk from persecution by the Laos government if they return there. Many were soldiers or family members of soldiers — the so-called “forgotten allies”– who decades ago fought in a secret army set up by the United States to combat the communist insurgents who eventually took over the country in 1975.

Read more: The Washington Post