The United States expressed disappointment Tuesday that Cambodia had demolished a second American-funded military facility without warning, as the kingdom turns to China to expand a naval base.

Cambodian authorities this month finished dismantling a maintenance facility at Ream base -- built in 2017 with US money -- after razing a tactical naval headquarters there in September.

Cambodia's Defence Minister Tea Banh confirmed the demolition to AFP on Tuesday, drawing a rebuke from the US embassy.

"We are disappointed that Cambodian military authorities have demolished another maritime security facility funded by the United States, without notification or explanation," embassy spokesman Chad Roedemeier told AFP.

The Ream base is strategically located in the Gulf of Thailand, giving access to the fiercely contested South China Sea -- a key global shipping route.

Beijing claims the majority of the resource-rich sea -- vying with six other countries.

The US said last month it fears the demolitions may be tied to plans for hosting Chinese military assets and personnel at the Ream base.

But Tea Banh insisted Cambodia was acting in its own interests and had no obligation to inform the US of its plans.

"We do this for Cambodia totally, not to serve anyone (else). Cambodia has the right to seek aid from anyone who wants to help Cambodia's development," he said.

The Wall Street Journal last year reported on a secret draft deal allowing China to dock warships at the Ream base.

The Cambodian government in June denied this and maintained its foreign policy was independent.

Cambodia said the facilities were simply being relocated about 30 kilometres (19 miles) north of Ream because they had outgrown the former sites.

Prime Minister Hun Sen has repeatedly insisted Cambodia's constitution forbids any foreign military base within its borders.

In recent years, he has tilted away from the US after Washington's criticism of alleged abuses by his government.