The U.S. military says two Chinese Sukhoi Su-30 fighter jets have intercepted a U.S. radiation-detection aircraft over the East China Sea.

The crew aboard the U.S. WC-135 plane characterized the move as "unprofessional," Air Force spokeswoman Lieutenant Colonel Lori Hodge (eds: a woman) said on May 18.

The issue is being addressed with China through "appropriate diplomatic and military channels," she added.

Hodge also said the U.S. plane was conducting a routine mission in international airspace in accordance with international law when the May 17 incident occurred -- the second between U.S. and Chinese planes this year.

One of the Chinese Su-30s came within 45 meters to the U.S. plane, CNN quoted a U.S. official as saying.

Chinese Foreign Ministry spokeswoman Hua Chunying declined to comment on this specific incident, but said, "For a long time, U.S. ships and aircraft have been carrying out close up surveillance of China which can really easily cause misunderstandings or misjudgments, or cause unexpected incidents."

The four-engine WC-135 Constant Phoenix aircraft has been used in the past to gather evidence of possible nuclear tests by North Korea.

China’s suspicion of U.S. activity in the South China Sea has repeatedly led to tensions between the two powers.

Based on reporting by Reuters, AP, CNN, and the BBC