The American plane, a WC-135 Constant Phoenix, collects samples from the air to detect nuclear explosions. The U.S. Air Force said it was on a routine mission in international airspace. An American official told CNN, however, that the plane has been regularly deployed in Northeast Asia to gather evidence of possible further nuclear tests by North Korea.

The Chinese military has become increasingly unhappy with American surveillance in the East China Sea and in Southeast Asia in the South China Sea, where Beijing claims full sovereignty in an ongoing dispute with U.S. allies in the region such as the Philippines.

Intercepts are not uncommon in both areas. But the latest incident appeared an unusually tight encounter.

Two Chinese SU-30 fighters flew up close to an American WC-135 on Wednesday, the U.S. Air Force said in a statement, saying the American aircrew had described the intercept as “unprofessional,” based on the Chinese pilot’s maneuvers and the speeds and proximity involved.