A written statement from the department said ousters were ordered by the Permanent Committee of the Cambodian National Assembly, made up entirely of members of the ruling party.
"We strongly support a political process that includes the full participation of all political parties on a level playing field," the statement said.
The BBC said the dismissal of the opposition members this week was quickly followed by passage of a measure in the assembly making it a crime to deny atrocities took place in Cambodia in the 1970s when the Khmer Rouge held power.
Human rights activists said the new law was a thinly disguised political move aimed at shoring up Prime Minister Hun Sen's re-election bid by demonizing opposition parties that have blamed Vietnam for some of the approximately 1.7 million Cambodians who died under the Khmer Rouge.
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