Sixteen members of the US Congress have signed a joint letter to Prime Minister Hun Sen calling on the premier to “immediately cease the harassment and persecution” of the opposition.

The letter, sent late last week, pointed to the National Assembly’s decision last month to expel Cambodia National Rescue Party president Sam Rainsy from parliament as of “gravest concern” to the US officials, which included longtime Cambodia National Rescue Party supporter Ed Royce, a Republican congressman.

“We call on you to immediately cease the harassment and persecution of Cambodia’s opposition, revoke … Rainsy’s arrest warrant and reinstate him to the National Assembly, renounce all forms of political violence, and foster an environment where democracy can thrive and flourish,” the letter reads.

The arrest warrant against Rainsy was filed last month over a long-standing defamation ruling involving allegations that Foreign Minister Hor Namhong committed crimes while jailed by the Khmer Rouge at a prison camp in the late 1970s.

Another summons was issued on November 20, linking the CNRP president to the case of imprisoned opposition Senator Hong Sok Hour.

“While we have been profoundly disappointed by widespread irregularities and fraud reported by national and international observers in the 2013 national elections that left the Cambodian People’s Party in power, we have been encouraged by negotiations between the ruling party and CNRP that resulted in opposition parliamentarians taking their seats,” the letter continues.

The representatives also pointed to the passing of a law on non-governmental groups and associations and the beating of two opposition lawmakers outside parliament last month as further evidence of a state-sanctioned crackdown on dissent.

Rainsy did not respond to a request for comment yesterday, but on his Facebook page on Saturday he repeated claims that the government’s moves against the opposition, which included still more charges against him being issued last week, were taking Cambodia back to a one-party state.

Government spokesman Phay Siphan dismissed the letter yesterday, saying the US officials were “trying to create a culture of impunity in Cambodia”, a claim often levelled at the government by the opposition and civil society.

“They [the letters’ authors] do not understand what is happening in Cambodia,” he said. “So the letter has no power.”

“Rainsy would do well to behave better and not [commit crimes].”

Namhong yesterday said it was the US officials’ right to express themselves, but added that Cambodia was acting constitutionally.

“Even if we had no aid, Cambodia would keep going forward and developing in all sectors,” he said. “I believe Cambodia will continue to improve and never go backwards.”