Washington's relationship with Phnom Penh has been deteriorating for years, with China pouring billions of dollars into infrastructure investment under Cambodia's former leader Hun Sen.

AFP reporters saw Cambodian navy personnel welcoming the USS Savannah's arrival to the southern port city of Sihanoukville on Monday.

The ship's commanding officer Daniel A. Sledz said "it is great to be back, returning US presence here after eight years", adding that his crew were "very pleased" to be hosted by Cambodia.

The US embassy posted a statement on social media on Monday saying the combat ship is on a "goodwill visit" that aims to "boost US-Cambodia coordination and response to shared maritime security challenges".

Cambodia's defence ministry said on Friday that the five-day visit aims to "strengthen and expand friendship" and "promote bilateral cooperation".

Since 2022, China has been funding the renovation of the Ream naval base, some 30 kilometres (20 miles) from Sihanoukville, which was originally built partly using US funds.

Washington has said the Ream base could give Beijing a key strategic position in the Gulf of Thailand near the disputed South China Sea, which China claims almost in its entirety.

Cambodian leaders have repeatedly denied that the base is for use by any foreign power.

Chinese warships first docked at the 363-metre (1,190-foot) pier in December last year. Two berthed at Sihanoukville port in May as part of Beijing's biggest joint military drills with Cambodia.

The Chinese military unveiled machine gun-equipped "robodogs" this year at the annual joint exercises, known as the "Golden Dragon" drills.

In early 2017, Cambodia scrapped similar joint exercises with US forces that had been held for the previous seven years.

Cambodia's defence ministry said 27 US navy vessels have visited the nation since 2007, although the USS Savannah's visit with 103 crew members was the first docking in eight years.

The ship's senior officers will meet with the Ream base's commander.

On Monday, Beijing responded to the US warship's visit to Sihanoukville saying "such exchanges and collaborations in security and defence should contribute to promoting regional peace and stability, rather than the opposite."

US Defense Secretary Lloyd Austin visited Cambodia in June to reset ties with the staunch China ally.

Cambodia's foreign minister Prak Sokhonn and the US embassy's Bridgette Walker last week hailed the "reinvigoration" of military ties between the two countries.