SINGAPORE - A decision by Cambodia to expand its biggest naval base and allow the Chinese military exclusive use of a portion of it shows an extraordinary lack of transparency, a senior U.S. defense official said on Thursday.

A Chinese official said on Wednesday an "iron-clad partnership" with Cambodia was bolstered by military cooperation, as work began on a China-funded upgrade of Cambodia's Ream naval base.

Cambodian Minister of Defense Tea Banh has dismissed fears that it would let China build a military base on its soil, saying any country could use the facilities at the Ream naval base, while Cambodia was open to accepting military assistance from anyone.

The United States believes that the expansion plans for the base included exclusive use of the northern portion of the base for China's military and neither country had shared full details about the extent of Beijing's plans to have a unilateral military base there.

"The lack of transparency has been extraordinary...(it is) in direct contravention of months and months of denial by them that there was any PRC involvement," the official, speaking on the condition of anonymity, said.

The official said the two countries had taken steps which were "bordering on the absurd" to hide Chinese military activity, including disguising Chinese personnel during visits by foreign officials to the base.

"What we are calling for, what the region is calling for, is more transparency, around the PRC activities and that lack of transparency has been cause for concern and cause for suspicion among countries around the region," the U.S. official added.

The comments come as U.S. Defense Secretary Lloyd Austin arrived in Singapore to meet with a number of his Asian counterparts, including an expected meeting with Chinese Minister of National Defence General Wei Fenghe.

The United States and China are expected to use Asia's top security meeting to trade blows over everything from Taiwan's sovereignty to the war in Ukraine, although both sides have indicated a willingness to discuss managing differences.

Cambodia-U.S. ties have been frayed for years over U.S. accusations that long-serving Prime Minister Hun Sen and his ruling party have stifled democracy though persecution of the opposition.

A year ago, Cambodia said it had razed a U.S.-funded facility at the naval base to allow for further expansion.

Underlining Cambodia's warm ties with China, Tea Banh this week posted pictures on social media of him taking a dip in the sea with China's ambassador to Cambodia, Wang Wentian.

But Tea Banh was also keen to allay fears in the United States and the region that Cambodia might open the door to China's military.

(Reporting by Idrees Ali; Editing by Kim Coghill)