A renewed campaign by Cambodian Prime Minister Hun Sen to silence critics of his more than 20-year rule has put in jeopardy signs of progress in the economy and in civil society, U.S. officials and leading human-rights groups warn.

The arrests of three leading Cambodian human-rights activists in Phnom Penh on charges of defaming the government is the latest sign of a slide toward increasing authoritarian rule in the Southeast Asian nation.

"We are concerned that this may be part of a broader plan to quash the opposition," U.S. Ambassador to Cambodia Joseph Mussomeli told reporters in Phnom Penh after the Dec. 31 detention of Kem Sokha, director of the Cambodian Center for Human Rights.

Mr. Mussomeli, who witnessed the arrest of Mr. Kem Sokha, said the government "has scared ... the opposition, and it becomes difficult to take these trappings of democracy as the real thing."

The crackdown has brought an unusually sharp warning from the World Bank that international financial support for Cambodia may be in jeopardy.

Still coming to terms with the social, political and economic devastation of the rule of the Khmer Rouge, the country finally appears to be benefiting from the economic boom that has swept the region. Although still among the world's poorest countries with per capita income of $320, Cambodia in recent years has showed an annual growth rate of about 7 percent in the gross domestic product.

Its two leading industries -- garment exports and tourism -- could be hurt by a political crackdown that angers Western donors and scares Western investors, said Ian Porter, the World Bank country director for Cambodia.

The arrests "will send a worrying message to potential investors and Cambodia's development partners," Mr. Porter said Monday.

Despite its economic growth, Cambodia receives an estimated $500 million annually in foreign aid. The next meeting of its donors is set tentatively for March.