GENEVA:The head of the UN refugee agency Antonio Guterres Thursday urged Thailand to call off plans to send more than 4,000 ethnic Hmong back to communist Laos, where many fear persecution.

"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," Guterres said.

"To proceed otherwise would not only endanger the protection of the refugees but set a very grave international example," said a statement.

The Thai government has said it will expel the Hmong being held in the northern Phetchabun province before the end of the year, as agreed with the Lao regime, despite international protests.

The group are seeking political asylum based on claims they face persecution from the Laotian regime because they fought alongside US forces during the Vietnam War.

Thailand says they are economic migrants and has refused access for the UN High Commissioner for Refugees to assess if any are in fact political refugees.

Nine US senators have written to Thai Prime Minister Abhisit Vejjajiva to protest the deportation and denounce the government's "lack of transparency" in screening the Hmong.

European diplomats have also met the government in recent days to express their concerns.

Although Thailand has said it would not use force to return the Hmong, there are reports the army has significantly boosted troop numbers in Phetchabun, further stoking fears Bangkok would abide by the December 31 deadline.