The United States is deploring the killing Thursday of a prominent Cambodian trade union leader, Chea Vichea. The State Department called on Cambodian authorities to bring those responsible to justice and not allow a "culture of impunity" to prevail.

The shooting death of Chea Vichea in Phomn Penh drew a strong condemnation from the State Department, where officials are concerned about a mounting toll of apparently-political killings in Cambodia since last year.

Mr. Vichea, president of the Cambodian Free Trade Union of Workers, was shot several times at a newsstand in the capital by an assailant who reportedly escaped on a motorcycle driven by an accomplice.

At a news briefing, State Department spokesman Adam Ereli called Mr. Vichea a champion of labor rights and the free-trade movement in Cambodia, which he said the United States strongly supports.

"We deplore this cowardly act of violence, and we call upon the Cambodian government to take immediate and effective action to bring the perpetrators of Chea Vichea's murder to justice. It's important that a culture of impunity in Cambodia not be allowed to exist," he said.

Spokesman Ereli said the United States urges "restraint on all sides" in the wake of the killing so that the tragedy of Mr. Vichea's death will not be compounded by further violence.

He also urged the Cambodian government to take steps to ensure the security of the labor leader's family, colleagues and other labor organizers.

Mr. Vichea was affiliated with Cambodia's main opposition political group, the Sam Rainsy party and was at least the fourth person linked to that party to be killed since the beginning of the month.

Last October, the State Department issued a similar, vigorous condemnation of the killing of a journalist for a radio station aligned with the royalist Funcinpec party, who was also killed by motorcycle-riding attackers.

Cambodia has been locked in a political struggle since last July, when Prime Minister Hun Sen's Cambodian People's Party won a general election but failed to get the two-thirds margin needed to govern alone.

The Sam Rainsy and Funcinpec parties have refused to join a coalition government unless Mr. Hun Sen steps down.

Scores of activists of the two opposition groups have been slain in violence in the run-up to and aftermath of the election, and members of the ruling party have been killed as well.

State Department spokesman Ereli said Cambodian leaders are continuing efforts to form a new government based on the July 27 election, and that it is essential they be able to do so "in an environment free of intimidation and violence."

He said all elements of Cambodian society have a responsibility to take a constructive role in settling conflicts peacefully.