BANGKOK (Reuters) - The U.S.-funded National Democratic Institute (NDI) and rights groups say they are disappointed by the muted international action ahead of a general election in Cambodia on Sunday which long-serving Prime Minister Hun Sen is expected to win easily.

Supporters of the ruling Cambodian People's Party (CPP) gather during an election campaign in Phnom Penh, Cambodia, July 7, 2018. REUTERS/Samrang Pring/File Photo

Cambodians will go to the polls in an election that critics have called a one-horse race after the Supreme Court ruled to dissolve the main opposition Cambodia National Rescue Party (CNRP) last year.

A month after the CNRP was dissolved, the United States said it would deny visas to Cambodian officials involved in suppressing the opposition -- the toughest steps by any Western country since Hun Sen launched his crackdown on critics.

The European Union and the United States, however, have not removed preferential trade access for Cambodia’s vital garments sector despite calls for more sanctions.

“The response of the international community has been disappointing,” John Cavanaugh, Cambodia Country Director, National Democratic Institute (NDI), told a news conference in Bangkok on Tuesday.

“I think one of the reasons for the muted international response is the international community might be caught up in a feeling of complacency because of the Hun Sen’s government pattern of applying pressure and then reverting back to normal,” he added.

“What we’re seeing is this is not going to revert back to the old normal, this is a new normal ... so we really are calling for a much more aggressive international response.”

Hun Sen has ruled Cambodia for over 30 years.

The CNRP was narrowly defeated in a 2013 general election, but the country’s once-thriving opposition has been silenced and pushed underground ahead of the general election.

Some have called for a boycott of the vote but authorities, including a spokesman for Hun Sen’s ruling Cambodian People’s Party (CPP) have said boycotting the vote is illegal and that action will be taken against those who urge others not to vote.

Phil Robertson, deputy director of Human Rights Watch’s Asia division, decried the lack of international pressure.

“There is no real excuse, frankly, for the failure of governments to speak out,” he said.