Two Chinese fighter jets conducted an “unprofessional” intercept of a U.S. radiation-detecting plane in international airspace over the Yellow Sea off the Korean Peninsula, the U.S. military said Friday.

The U.S. aircraft, a U.S. WC-135 Constant Phoenix, known colloquially as a “nuke sniffer,” was intercepted Wednesday by two Chinese Su-30 fighters during what the U.S. Air Force termed “a routine mission.”

“The WC-135 was operating in accordance with international law,” air force spokeswoman Lt. Col. Lori Hodge said. “While we are still investigating the incident, initial reports from the U.S. air crew characterized the intercept as unprofessional.”

Hodge said the intercept is being addressed with Beijing via appropriate diplomatic and military channels.

The Chinese jets came within 150 feet (45 meters) of the U.S. plane — with one of the Su-30s flying inverted, or upside down, directly above the American plane — CNN reported, citing unidentified U.S. defense officials.

RELATED STORIES China, ASEAN agree on code of conduct framework for disputed South China Sea

The Pentagon has often deployed one of the air force’s two WC-135 aircraft to the Asia-Pacific region after North Korea’s first underground nuclear test in October 2006 in an apparent effort to detect atomic tests by the reclusive regime in Pyongyang.

The aircraft uses external flow-through devices that collect air and debris samples that are later sent to a lab for analysis.

A WC-135 was deployed to the U.S. Air Force’s Kadena Air Base, in Okinawa, last month amid rising tensions with the North as the nuclear-armed country marked two key anniversaries with a spate of missile tests.

While intercepts such as Wednesday’s are rare, a U.S. Navy reconnaissance aircraft and a Chinese military plane were also involved in an “unsafe” encounter in international airspace over the disputed South China Sea in February.

In that incident, the two planes flew within 1,000 feet (300 meters) of each other in the area of the contested Scarborough Shoal, just 230 km (140 miles) from the Philippine coast, media reports said.

Beijing and Washington have been in talks about ways to curb North Korea’s nuclear weapons and missile development program, and the intercept could add another divisive element to already fraught discussions.

Joshua Pollack, editor of the U.S.-based Nonproliferation Review, said China may have mistaken the WC-135 for another U.S. spy plane, but noted that Beijing is extremely sensitive about American military flights in the area.

“In general, they don’t like to see the U.S. military in the Yellow Sea and throw brushback pitches against recon aircraft from time to time,” Pollack said.

“They think of it as their sea,” he said. “Notice that the North Koreans have never shot a missile into those waters.”