The United States has said Ream Naval Base, off Cambodia's southern coast, could give China a key strategic position in the Gulf of Thailand near the disputed South China Sea, which Beijing claims almost in its entirety.

The littoral combat ship USS Cincinnati (LCS-20) docked Saturday morning at one of the base's piers 150 metres (yards) away from a pair of Chinese warships.

"It is our privilege and our honour to be here as the first US naval vessel to moor pierside at Ream Naval Base, and we hope this is the beginning of a longstanding tradition and friendship," Andrew J. Recame, the ship's commanding officer, told reporters.

Cambodian leaders have repeatedly denied that the base is for use by any single foreign power, following US media reports in 2022 saying the new facilities at Ream -- originally built partly with US funds -- would be exclusively for the Chinese navy.

Ream base said in a statement that the five-day US visit would "promote cooperation between the two countries", and that it showed Cambodia's "commitment in implementing an open policy, transparent and cooperation with international partners".

Cambodian Prime Minister Hun Manet and a delegation from China's People's Liberation Army inaugurated the base in April last year.

Hun Manet denied the new and improved facility would be for Beijing's "exclusive" use, saying ships from other countries would be allowed to dock.

Two weeks after its inauguration, two Japanese warships were the first vessels to dock at the base.

Beijing has since 2022 been contributing to a revamp of the Ream Naval Base, which was originally built partly using US funds.

Western concerns about the base go back as far as 2019, when The Wall Street Journal reported on a secret draft deal allowing China to dock warships there.

In late 2023, Chinese warships first docked at the 363-metre (1,190-foot) pier, on Cambodia's sole coastline in the south of the country between Thailand and Vietnam.

A US warship docked in the commercial Sihanoukville port in 2024 in the first American military port call in Cambodia in eight years.

On Saturday morning, AFP journalists saw two Chinese warships still docked at the base.

Cambodia has long been one of China's staunchest allies in Southeast Asia, and Beijing has extended its influence over Phnom Penh in recent years.

Under former leader Hun Sen -- Prime Minister Hun Manet's father -- China poured billions of dollars into infrastructure investments, while Washington's relationship with Phnom Penh has deteriorated in recent years.