Washington, May 19 (IANS): Two Chinese Su-30 fighter jets intercepted an American Air Force radiation detection plane over the East China Sea, according to the US Pacific Air Forces.

A US official told CNN on Thursday that the Chinese jets came within 150 feet of the US Air Force WC-135 plane, with one of the Su-30s flying inverted, or upside down, directly above the American plane.

The US crew aboard the WC-135 characterised Wednesday's move as "unprofessional", a statement from Air Force Lt. Col. Hodge said.

"While we are still investigating the incident, initial reports from the US aircrew characterized the intercept as unprofessional. The issue is being addressed with China through appropriate diplomatic and military channels," Hodge said.

Dubbed the "Constant Phoenix," the four-engine WC-135 jet looks for distinctive elements a nuclear test of any type would emit into the air. The collected samples can be analyzed to determine exactly what occurred, reports CNN.

The WC-135 has been regularly deployed on routine missions in Northeast Asia, according to the official.

The planes have been used in the past to gather evidence of possible nuclear tests by North Korea.

Wednesday's incident was the second between US and Chinese planes this year.

In February, US defence officials said there was an "unsafe" close encounter between a US Navy P-3 Orion aircraft and a Chinese surveillance aircraft over the South China Sea.