China has rejected a protest from Vietnam over a flight test it has conducted on a new airstrip on a man-made reef in the South China Sea, saying it is part of China's territory.

The Chinese Foreign Ministry said Sunday that it conducted a civil flight test on the newly built airstrip on Fiery Cross Reef in the Spratly Islands, a chain of islets rich in natural resources that are the focal point of rival claims by neighboring governments.

China has become more assertive in pressing its claims in the South China Sea, and has recently piled sand on seven features atop of which it built airfields, radar installations and docking facilities.

The Foreign Ministry says Vietnam's protests over the flight test are unreasonable.