Two Chinese SU-30 aircraft carried out what the U.S. military described as an “unprofessional” intercept of a U.S. aircraft designed to detect radiation while it was flying in international airspace over the East China Sea.

“The issue is being addressed with China through appropriate diplomatic and military channels,” said Air Force spokeswoman Lieutenant Colonel Lori Hodge. She added that the aircraft was carrying out a routine mission and was operating in accordance with international law.

China declined to comment on the issue.