By Prak Chan Thul

PHNOM PENH (Reuters) - A U.N.-backed war crimes tribunal in Cambodia dismissed charges against a former cadre of the 1970s Khmer Rouge regime on Wednesday, saying the Buddhist nun had not played a senior enough role during a period when some 1.8 million people died.

The decision in the case of Im Chaem, who was suspected of running a forced labor camp, was a boon for veteran Prime Minister Hun Sen, a former Khmer Rouge soldier who opposes further trials.

Critics said the decision undermined the credibility of the court, which has found just three people guilty after a decade of work at a cost of over $260 million.

The Extraordinary Chambers in the Courts of Cambodia said in a statement that Chaem, in her 60s, did not fall under their jurisdiction because she was not a senior leader of the Khmer Rouge or one of the most responsible officials.

Most of the victims of the Khmer Rouge died of starvation, torture, exhaustion or disease in labor camps or were bludgeoned to death during mass executions. Pol Pot, "Brother Number One," died in 1998.

Chaem, a former Khmer Rouge district commander, was charged with murder and crimes against humanity.

She did not appear in court because Cambodian police would not arrest her or other senior Khmer Rouge cadres charged. Hun Sen, in power for 30 years, has warned that more trials could see Cambodia spiral into civil war.

"As long as the judges followed the rules and the evidence, we must accept the decision, but it can be difficult sometimes to swallow," said Youk Chhang, who said he suffered at Im Chaem's camp at the age of 15.

His team has spent more than 20 years documenting the horrors of the regime and provided half a million documents to the tribunal.

The court faced a test of its credibility and would need to explain its decision to survivors, said Panhavuth Long of the Cambodian Justice Initiative, which monitors the trials.

"We've seen concerns by people in general about interference," he told Reuters.

The court has been plagued by infighting, political interference, resignations and funding shortages since it was set up to bring to justice "those most responsible" for the deaths of a fifth of the population from 1975-1979.

(Editing by Matthew Tostevin)