Hun Sen (File photo)

Prime Minister Hun Sen may ask Thailand to extradite two Cambodian activists, who fled the country to evade arrest for allegedly defaming him. They left after another activist was jailed on Saturday, the second arrest of a critic of the prime minister in just two weeks.

The Cambodian prime minister said Monday he might call on the Thai government to help find and return two men wanted for questioning in Cambodia over charges of criminal defamation and inciting others to commit criminal acts.

Men Nath and Ear Channa, members of a coalition of unions critical of the government, went into hiding on Saturday after a fellow activist was jailed on similar charges. A third union leader wanted for questioning is in Europe.

The men, leaders of the Cambodia Watchdog Council of student, teacher, garment factory and civil servant unions, attacked Hun Sen's signing of a controversial supplementary border agreement with Hanoi last Monday. They threatened to stage a hunger strike to protest the accord. The council joined a chorus of concern over the loss of Cambodian territory to its former occupier, Vietnam.

Vietnam troops overthrew Cambodia's murderous Khmer Rouge regime in 1979, and occupied the country until 1991.

Hun Sen, who was installed by Vietnam as Cambodia's foreign minister in 1979 and became prime minister in 1985, last week promised to sue anyone who accused him of selling Cambodian territory.

Sok Sam Oeun, executive director of the legal aid program, the Cambodian Defenders' Project, says the government's use of defamation lawsuits has created a culture of fear among civic organizations.

"All NGO leaders, they are very scared now. So, it also impacts to the right of expression," he said. "They cannot express anything what they want."

Sok Sam Oeun said some civic leaders have left the country, or are planning to go soon.

Rong Chhun, a leader of the Cambodia Watchdog Council and president of a national teacher's union, was arrested on Saturday, as he attempted to cross the border to Thailand. He has been charged with defamation and incitement, and could face up to five years in prison.

His detention came just a week after a popular radio journalist was jailed, also on charges of criminal defamation, for airing an interview critical of the prime minister.

The U.S. Embassy in Phnom Penh has called on the Cambodian government to respect the constitutionally protected rights of free press and expression.

At an economic forum on Friday, the prime minister told foreign investors, donors and diplomats to stay out of Cambodia's internal affairs. He said the country always has a few troublemakers, but that they would soon be in prison, and life would return to normal.