The U.N.-backed tribunal trying members of Cambodia's former Khmer Rouge regime for genocide and other crimes says it has formally charged the group's navy commander, Meas Muth.

Prime Minister Hun Sen has opposed any expansion of the trial, and Cambodian police had refused to arrest Meas Muth.

The tribunal said Meas Muth appeared voluntarily to hear the charges against him.

The tribunal operates under a system which pairs international and Cambodian judges, prosecutors and lawyers. Three defendants have been found guilty in earlier trials, but Cambodian officials have not cooperated in prosecuting new suspects.

The charges against Meas Muth include homicide, genocide, crimes against humanity and other offenses.

Hun Sen was a mid-level Khmer Rouge commander before defecting while the group was still in power.