KAMPONG CHHNANG, Cambodia – China's military showed off its machine gun-equipped robot battle 'dogs' on Thursday at the start of its biggest-ever drills with Cambodian forces, as the United States frets about Beijing's growing influence in the Southeast Asian country.

Cambodia has long been a staunch ally of China, receiving billions of dollars in investments, and Washington is growing increasingly concerned that Beijing will use a Cambodian naval base it is upgrading on the Gulf of Thailand to expand its influence in the region.