By Prak Chan Thul

PHNOM PENH (Reuters) - China is to help Cambodia revamp its judiciary, a system that opposition party supporters say is in thrall to Prime Minister Hun Sen, and that the United States has criticized as "politicized and ineffective".

The two sides signed a memorandum of understanding on Chinese assistance for Cambodia's courts this week, Ministry of Justice spokesman Chin Malin said on Wednesday, adding that China would provide training, finance and expertise.

"There will be training and exchanges of visits between the two countries' officials, at a level up to technician and minister," Chin Malin told Reuters.

China would also provide expertise to improve Cambodia's commercial courts, he said.

"Assistance for the judiciary is like promoting human rights ... Once, there is a good legal system, it promotes respect for human rights."

China is Cambodia's closest ally.

Relations between Cambodia and Western powers including the United States and European Union, are frequently strained by various differences including over human rights.

Cambodia has backed China on its territorial claims in the South China Sea to the disappointment of some of Cambodia's neighbors, who have rival claims with China.

The U.S. State Department said in a 2015 report on rights in Cambodia that a "politicized and ineffective" judiciary was among Cambodia's most significant human rights problems.

The government didn't respect judicial independence and the courts "were subject to influence and interference", it said.

The judiciary has come in for particular criticism recently over cases against opposition leaders that they say are politically motivated by Hun Sen's government in the run-up to what is expected to be a close general election in 2018.

The government and judiciary reject accusations of bias.

The independent Cambodian Center for Human Rights said that given a lack of judicial independence in China, it was difficult to see how the agreement would address Cambodia's chronic problems.

"While technical assistance is required ... the involvement of China may serve to further entrench the endemic lack of judicial independence in Cambodia's courts," said the center's director, Sopheap Chak.

"Without a true separation of powers, any reform of Cambodia's courts will be meaningless," she said.

(Reporting by Prak Chan Thul; Editing by Robert Birsel)