BANGKOK—The Thai military has mobilized troops and buses and was preparing Sunday to forcibly return 4,000 Hmong asylum seekers to Laos in a lingering echo of the Vietnam War, human rights groups and other observers said.

Members of a mountain tribe that aided the United States in its secret war in Laos, the asylum seekers say they fear retribution by the Laotian government, which continues to battle a ragged insurgency of several hundred Hmong fighters.

Thailand appears to be moving ahead with the repatriation despite complaints from the United States, the United Nations and human rights and aid groups. It is doing so although it has determined that some asylum seekers were eligible for refugee status, human rights groups said.

“;This forced repatriation would place the refugees in serious danger of persecution at the hands of the Lao authorities, who to this day have not forgiven the Hmong for being dedicated allies of the United States during the Vietnam War,”; Joel R. Charny, acting president of Refugees International, an advocacy group in Washington, said in a statement.

The remote Hmong encampment in Phetchabun province, about 200 miles north of Bangkok, is a remnant of an Indochinese refugee population that once numbered 1.5 million. That included boat people from Vietnam, survivors of the brutal Khmer Rouge government in Cambodia and hundreds of thousands of Hmong who crossed the Mekong River from Laos.

Since the war ended in 1975, the United States has processed and accepted about 150,000 Hmong refugees in Thailand for resettlement in the United States. But in the past three years Thailand has not allowed foreign governments or international agencies to interview the Hmong.

Refugee experts say the camp residents are a mix of refugees who fear persecution and economic migrants who have left Laos over the past few years. They have included dozens who display what appear to be battle scars, as well as some older refugees who fought on the American side during the war.

A separate group of 158 asylum seekers has been interviewed by the United Nations, which has labeled them “;people of concern”; who could face persecution if returned. But the Thai government says these asylum seekers will be forcibly repatriated eventually.

A government spokesman, Panitan Wattanayagorn, said that the exact timing of the deportations was in the hands of the military, but that it would be completed by Thursday, in accordance with an agreement with Laos.

He said Laos had said that the returnees would be treated well and that the United Nations could interview them within 30 days of arrival to determine if any were eligible for resettlement elsewhere. “;There is no reason to believe that they will be harmed,”; he said.

Reporters have not been permitted into the camps since 2007, and last May the main aid group assisting the Hmong in Phetchabun, Medecins Sans Frontieres withdrew from the camp in protest of the conditions there.

“;We can no longer work in a camp where the military uses arbitrary imprisonment of influential leaders to pressure refugees into a 'voluntary' return to Laos, and forces our patients to pass through military checkpoints to access our clinic,”; the group said.

On Sunday, Sunai Phasuk, the Thailand representative of Human Rights Watch, said a joint task force under military command had been assembled at the camp to repatriate the residents. He said the security forces had been instructed to wear body armor in case of violent resistance, which has accompanied forced returns in the past.

He said that Maj. Gen. Thanongsak Apirakyothin, the third army regional commander, arrived at the camp on Sunday and that the army was preparing to send everyone back to Laos.

Speaking by telephone from Washington on Sunday, Eric P. Schwartz, assistant secretary of state for population, refugees and migration, said that he had met with high-level officials in Thailand last week and that the United States was prepared to assist both with questions of third-country asylum and with the return to Laos of economic migrants. He said Thailand had rejected this offer.

“;We recognize the challenge of irregular migration that the government of Thailand faces, but there is absolutely no need to resort to these kinds of measures,”; he said.

Lionel Rosenblatt, president emeritus of Refugees International and a key figure in the planning for the postwar evacuations from Vietnam, Cambodia and Laos, said Thailand had been an active transit point for as many as 1.5 million refugees from the wars in Indochina.

He said that if the deportations proceeded, they would mar Thailand's otherwise positive record in assisting with the resettlement of refugees.