In addition to lodging diplomatic protests, Hanoi filed a complaint with the United Nations’ International Civil Aviation Organization, accusing Beijing of flying planes without warning into an air-traffic control area supervised by Vietnamese aviation authorities.

Vietnam is mandated by ICAO to supervise international air traffic through a large portion of the South China Sea known as the Ho Chi Minh City Flight Information Region. Vietnamese aviation authorities said they recorded 46 instances of Chinese planes flying without warning through that area in the first week of January, including the test flights to Fiery Cross Reef.

Speaking at a regular news briefing, Mr. Hong denied that the test flights have affected regional safety. He said that instead, they show Fiery Cross Reef is ready to handle civil-aviation operations that can benefit maritime search-and-rescue and other public services in the area.

He dismissed Vietnam’s protest to ICAO, saying the U.N. agency had no jurisdiction over the matter. He said the “Chinese government decided to designate the relevant flight activities into state aviation activities, to be conducted with civil aircraft.”

Spokesmen for Vietnam’s civil-aviation regulator and ICAO didn’t immediately respond to requests for comment.

Aviation experts said the dispute centers on whether China’s test flights—carried out using state-owned commercial airliners—qualified as “state flights.”

“It would be easy for a country to claim a state flight when it was not, and equally easy for other states to claim that the ICAO convention was being abused,” said David Learmount, a consulting editor at Flightglobal, an aviation industry publication.

The Convention on International Civil Aviation, of which China is a signatory, said its rules don’t apply to “state aircraft,” which are defined as “aircraft used in military, customs and police services.” The convention said signatory states will show “due regard for the safety of navigation of civil aircraft” when issuing regulations for their state aircraft.