The Chinese navy warned a U.S. surveillance plane flying over artificial islands that Beijing is creating in the disputed East Vietnam Sea to leave the area eight times, according to CNN, which was on board the flight on Wednesday.

At one stage, after the American pilots responded by saying the plane was flying through international airspace, a Chinese radio operator said with exasperation: "This is the Chinese navy ... You go!"

The P8-A Poseidon, the U.S. military's most advanced surveillance aircraft, flew at 15,000 feet (4,500 metres) at its lowest point, CNN said.

The incident, along with recent Chinese warnings to Philippine military aircraft to leave areas around Vietnam’s Truong Sa (Spratly) archipelago in the East Vietnam Sea, suggests Beijing is trying to enforce a military exclusion zone above its new islands.

Some security experts worry about the risk of confrontation, especially after a U.S. official said last week the Pentagon was considering sending military aircraft and ships to assert freedom of navigation around the Chinese-made islands.

Footage taken by the P8-A Poseidon and aired by CNN showed a hive of construction and dredging activity on the new islands the plane flew over, as well as Chinese navy ships nearby.

CNN said it was the first time the Pentagon had declassified video of China's building activity and audio of challenges to a U.S. aircraft.

"We were just challenged 30 minutes ago and the challenge came from the Chinese navy," Captain Mike Parker, commander of U.S. surveillance aircraft deployed to Asia, told CNN aboard the flight.

"I'm highly confident it came from ashore, this facility here," Parker said, pointing to an early warning radar station on Fiery Cross Reef.

Military facilities on Fiery Cross Reef, including a 3,000-metre (10,000-foot) runway, could be operational by year's end, one U.S. commander recently told Reuters.

Asia's rising power China claims sovereignty over most of the East Vietnam Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.

Foreign Minister Wang Yi last week asserted Beijing's sovereignty to reclaim the reefs, saying China's determination to protect its interests was "as hard as a rock".

China has also said it had every right to set up an Air Defence Identification Zone (ADIZ) in the East Vietnam Sea but that current conditions did not warrant one.

ADIZs are used by some nations to extend control beyond national borders, requiring civilian and military aircraft to identify themselves or face possible military interception.

During the P8-A mission, the pilot of a Delta Air Line flight in the area spoke on the same frequency after hearing the Chinese challenges, and identified himself as commercial. The Chinese voice reassured the pilot and the Delta flight went on its way, CNN said.