Cambodia has long been a staunch ally of China, receiving billions of dollars in investments, and 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.

More than 2,000 troops, including 760 Chinese military personnel, are taking part in the drills at a remote training centre in central Kampong Chhnang province and at sea off Preah Sihanouk province.

The 15-day exercise, dubbed Golden Dragon, also involves 14 warships -- three from China -- two helicopters and 69 armoured vehicles and tanks, and includes live-fire, anti-terrorism and humanitarian rescue drills.

The hardware on show included the 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.

Opening the exercises, Cambodian armed forces commander-in-chief Vong Pisen said they would "enhance the capabilities" of the two armies in the fight against terrorism.

- US worries -

Vong Pisen insisted Cambodia would never allow a foreign military base on its territory, echoing previous assertions by Cambodian leaders.

After Cambodia dismantled facilities at Ream naval base near the Cambodian port city of Sihanoukville, built partly with American funding and having played host to US military exercises, China began funding its renovation.

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 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.

Earlier this week, Cambodian army spokesman Thong Solimo told reporters the 2024 exercises were biggest ever of their kind and China would cover the cost.

The first Golden Dragon drills were held in 2016, and in early 2017 Cambodia scrapped a similar joint exercise -- "Angkor Sentinel" -- which had been held for the preceding seven years with US forces.

Cambodia's defence ministry last week confirmed the two Chinese warships anchored at Ream were "to help train Cambodian naval personnel and to prepare for the Golden Dragon exercises".

A spokesman said the Chinese vessels were testing the "Ream Naval Base that China is constructing for Cambodia", and denied Chinese troops would be stationed at the base.

A third Chinese warship docked in Sihanoukville on Monday with troops and materials for the exercises, according to the Cambodian army.

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.