A pair of Chinese fighter jets conducted an “unprofessional” intercept of an American radiation-sniffing surveillance plane over the East China Sea, the U.S. Air Force said on Friday, the latest in a series of such incidents that have raised U.S. concerns in an already tense region.

On Wednesday, the two Chinese SU-30 jets approached a WC—135 Constant Phoenix aircraft conducting a routine mission in international airspace in accordance with international law, Pacific Air Forces spokeswoman Lt. Col. Lori Hodge said in a statement.

The WC—135 crew characterized the intercept as unprofessional “due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft,” Lt. Col. Hodge said.

She declined to provide further details and said the issue would be addressed with China through “appropriate diplomatic and military channels.”

“We would rather discuss it privately with China,” Hodge said in an email to The Associated Press. “This will allow us to continue building confidence with our Chinese counterparts on expected maneuvering to avoid mishaps.”

China declared an air defense identification zone over a large section of the East China Sea in 2013, a move the U.S. called illegitimate and has refused to recognize.

China has demanded foreign aircraft operating within the zone declare their intentions and follow Chinese instructions. Lt. Col. Hoge declined to say whether Wednesday’s incident was within the self-declared Chinese zone.

“U.S. military aircraft routinely transit international airspace throughout the Pacific, including the East China Sea,” she said. “This flight was no exception.”

Unexpected and unsafe intercepts involving U.S. and Chinese military aircraft have occurred occasionally over the South China Sea, which China claims virtually in its entirety. Although China says it respects freedom of navigation in the strategically vital area, it objects to U.S. military activities, especially the collection of signals intelligence by U.S. craft operating near the coast of its southern island province of Hainan, home to several military installations.

In recent years, the sides have signed a pair of agreements aimed at preventing such encounters from sparking an international crisis, as happened in April 2001 when a Chinese jet fighter collided with a U.S. surveillance plane over the South China Sea, leading to the death of the Chinese pilot and China’s detention of the 24 U.S. crew members for 10 days.