The United States has said the Ream Naval Base, located 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.

Cambodian senior officials 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 base -- originally built partly with US funds -- would be exclusively for the Chinese navy.

Cambodian Prime Minister Hun Manet and a delegation from China's People's Liberation Army inaugurated the base early this month.

Hun Manet said during the opening that the base had "nothing to hide" and denied the new and improved facility would be for Beijing's "exclusive" use, saying ships from other countries would be allowed to dock.

Media access to the base was restricted on Saturday when the Japanese vessels Bungo and Etajima from the Japanese Maritime Self Defense Force (JMSDF) made a port call.

"We are glad to witness the visit to the renovated Ream Naval Base," the Embassy of Japan in Phnom Penh said in a statement sent to media, adding that it is "a historically significant event".

"We are aware that the Royal Government of Cambodia has been willing to make the base open to all the countries," the embassy said, adding that the visit would "enhance trust and confidence".

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.

Chinese President Xi Jinping this week wrapped up a tour of Southeast Asia, ending in Cambodia, in which he sought to strengthen regional trade ties.

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