When Defense Minister Tea Banh addressed graduates last month at Cambodia's prestigious Army Institute, he directed his thanks to the guests who made it all possible: a group of crisply dressed officers from China's People's Liberation Army.

The institute, established in 1999 around 80 km from Phnom Penh, is part of China's rising military aid to Cambodia. Interviews with serving officers and a senior Cambodian government official shed light on how far the school's influence has grown in recent years.

Military aid, alongside arms sales and billions of dollars of investment, have strengthened China's ties with Cambodia, and analysts see it as part of a push to extend regional influence, including in the disputed South China Sea.