Japan protests again after Chinese ships sail into waters near disputed isles
A group of disputed islands known as Senkaku in Japan and Diaoyu in China. Reuters

Two Chinese fighter jets have conducted an "unprofessional" intercept of a US aircraft, the US military said on Thursday. The aircraft was designed to detect radiation and it was flying in international airspace over the East China Sea.

"The issue is being addressed with China through appropriate diplomatic and military channels," Air Force spokeswoman Lieutenant Colonel Lori Hodge told Reuters.

Hodge said that the US characterization of the incident was based on initial reports from the US aircrew aboard the WC-135 Constant Phoenix aircraft "due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft."

"Distances always have a bearing on how we characterize interactions," Hodge added. She further said that a U.S. military investigation into the intercept was underway.

CNN cited US officials as saying that one of the Chinese jets came as close as 150ft (45m) to the US WC-135 plane and flew upside down above it.

According to Hodge, the WC-135 was carrying out a routine mission at the time and was operating in accordance with international law.

China is deeply suspicious of any US military activity in the resource-rich South China Sea.

On Feb 8, a US Navy P-3 spy plane and a Chinese military aircraft came quite close to each other over the South China Sea. This incident was interpreted as unsafe and also inadvertent by the Navy.

Reuters reported at the time that the aircraft came within 1,000 feet (305 meters) of each other in the vicinity of the Scarborough Shoal, between the Philippines and the Chinese mainland.