Vietnam has accused China of violating its sovereignty by landing a plane on an airstrip on an artificial island in a contested part of the South China Sea.
Le Hai Binh, foreign ministry spokesperson, said on Saturday the airfield had been "built illegally" on Fiery Cross Reef in the Spratly archipelago, in territory that was "part of Vietnam's Spratlys".
China's foreign ministry rejected the complaint, arguing that the flight was a matter "completely within China's sovereignty", the state news agency Xinhua reported.
Hua Chunying, the ministry's spokesperson, said the country used a civil aircraft to conduct the flight to test whether the airfield facilities meet civil-aviation standards.
The US said it was concerned that the flight had exacerbated tensions.
The US has criticised China's construction of artificial islands in the South China Sea and worries that China plans to use them for military purposes, even though China says it has no hostile intent.
Hanoi's foreign ministry said Vietnam handed a protest note to China's embassy and asked China not to repeat the action.
It called the flight "a serious infringement of the sovereignty of Vietnam on the Spratly archipelago".
China claims almost all the South China Sea, which is believed to have huge deposits of oil and gas, and through which about $5 trillion in ship-borne trade passes every year, and has been building up facilities on the islands it controls.
It completed an airfield on Fiery Cross Reef that security experts say could accommodate most Chinese military aircraft late last year.
Brunei, Malaysia, the Philippines and Taiwan also have rival claims in the South China Sea.