US officials suspect the Ream base is being converted for use by China, a close ally of Cambodia, as Beijing seeks to buttress its international heft with a network of naval and military outposts.

Cambodia has long denied the base on the Gulf of Thailand is for use by any foreign power -- despite Beijing funding the revamp -- but the row has been a festering sore in ties with the United States for years.

"It is near completion. We will inaugurate it soon," Chhum Socheat, a Cambodian defence ministry spokesman, told AFP

"We already declared that there is no Chinese military base there -- we are just modernising our military to reach a capable level in order to protect our territorial integrity."

New satellite images from BlackSky, a US commercial imagery company, show rapid progress in building work at the site over the past two years.

The pictures show a pier 363 metres (1,190 feet) in length jutting into the sea, according to an expert quoted by BlackSky.

"There is a near-exact similarity between an angled deep-water pier located on the western shore of the Ream base and another military pier at the People's Liberation Army Support Base in Djibouti," said the expert, Craig Singleton of the Foundation for Defense of Democracies, a hawkish Washington security think tank.

The Cambodian defence ministry spokesman rejected the claims about the pier being built to berth aircraft carriers.

US fears about the base go back as far as 2019, when the Wall Street Journal reported on a deal allowing Beijing to dock warships at Ream.

Cambodia has grown close to China under longtime ruler Hun Sen, whose party claimed a landslide victory in a weekend election condemned by Western powers as neither free nor fair.

Chinese President Xi Jinping sent a personal message to Hun Sen on Tuesday congratulating him on the election win and pledging to deepen ties further.

The 70-year-old, who has ruled Cambodia with an iron fist for nearly four decades, has indicated that he plans to hand power soon to his eldest son.