A senior U.S. State Department official visiting Cambodia defended U.S. support for Cambodia's military on Sunday. His comments follow strident criticism by a leading opposition parliamentarian over U.S. support for Cambodia's military.

The Under-Secretary of State for Political Affairs, William J. Burns, was in Cambodia this weekend to return seven looted antiquities recovered by U.S. officials.

Burns's visit coincides with the 60th anniversary of the establishment of U.S.-Cambodian diplomatic ties.

It also comes during a two-week military exercise in Cambodia involving more than 1,000 troops from 23 Asia-Pacific nations and the United States. It is part of the Global Peace Operations Initiative, a U.S.-run effort to improve peacekeeping skills among other nations.

But some Cambodian military units, including the tank unit hosting the provincial exercise, stand accused of human rights abuses.

Earlier this month Human Rights Watch said it was "outrageous" that the United States was supplying millions of dollars of equipment to army units, and undermined U.S. protests against forced evictions and land-grabbing.

Some local politicians are also displeased. Opposition M.P. Mu Sochua condemned American support for the Cambodian military.

"This is a huge insult to the people of Cambodia," said Mu Sochua. "This is not about helping democracy in Cambodia - this is about serving the US interests in the region."

At a news conference on Sunday, Burns stressed that all of America's military relationships were consistent with U.S. law, and said his government carefully vetted all participants.

But Mu Sochua said Washington was evading its responsibility. She said the U.S. Department of Defense had lied when it told Congress that none of Cambodia's military units were guilty of human rights violations.

"I am extremely disappointed by President Obama for allowing this to happen in Cambodia," she added.

Mu Sochua said donors, who earlier this year pledged more than $1 billion to Phnom Penh, must tie aid to human rights and democracy.

She said failing to do so meant the international community was failing in its obligations to Cambodia.

Mu Sochua's comments came Thursday as she awaited possible arrest for refusing to pay a fine levied in a defamation case filed by Cambodian Prime Minister Hun Sen.

The court fined her $4,000 after ruling that she had defamed Hun Sen by announcing her plan to sue him for defamation over comments he had made.

Her case was thrown out of court, but Hun Sen went on to win his.

She said the case was highly political, and was further evidence that democracy was being undermined by the ruling party.

William Burns alluded to her case when he said the political arena was a better venue for resolving such disputes than the courts. Freedom of expression, said Burns, was "an essential value for any healthy political society".