Two Chinese warships docked in Cambodia on Sunday as part of the biggest-ever joint military drills with the Southeast Asian nation, with Beijing's ambassador hailing their "ironclad" friendship.

Cambodia has long been a staunch ally of China and has received billions of dollars in investment.

Washington is growing increasingly concerned Beijing will use a Cambodian naval base it is upgrading on Gulf of Thailand to expand its influence in the region.

On Sunday the training ship Qijiguang and amphibious warfare ship Jinggangshan docked at Sihanoukville as part of 15 days of land and sea drills involving 760 Chinese military personnel along with around 1,300 Cambodians and 11 Cambodian vessels.

The Qijiguang bore a banner reading "Bring peace and friendship to meet good friends" as it approached the port.

"We are ironclad friends," Chinese ambassador Wang Wentian told reporters on the shore.

China-Cambodia military cooperation was "conducive to the security of both countries and to the security of the region."

At Ream, a few kilometres along the coast, China has been funding the renovation of a naval base originally built partly using US funds.

Two Chinese warships docked at Ream in December for the first time after work began to expand the base.

Washington says Ream could give Beijing a key strategic position on the Gulf of Thailand near the disputed South China Sea, which China largely claims.

Last year, Cambodian officials denied a new 363-metre (1,190-foot) pier at Ream was intended to berth aircraft carriers.

"Where the Chinese navy goes, we bring friendship, we bring cooperation, no others," ambassador Wang said in response to a question on concerns over Beijing's growing influence.

At the start of the drills on Thursday China's military showed off its hardware including so-called "robodogs" -- remote-controlled four-legged robots with automatic rifles mounted on their backs.

Handlers kept the dogs of war on the leash, demonstrating only their walking capabilities to watching journalists and top brass -- not their shooting skills.

The drills follow a three-day visit by China's top diplomat Wang Yi to Cambodia in April to deepen ties between the two countries.