The Chinese Sukhoi Su-30 jets reportedly came within close proximity of an American four-engine WC-135 jet, with one of the Su-30s flying upside down, directly above the American plane, Lieutenant Colonel Lori Hodge, a spokesperson for the US Air Force, said on Thursday.

The Chinese fighter jets had outmaneuvered the US jet, forcing it to descend hundreds of feet from its position.

“The issue is being addressed with China through appropriate diplomatic and military channels,” Hodge said.

The crew of the US Air Force WC-135 Constant Phoenix aircraft, also known as a “nuke sniffer,” described the confrontation as “unprofessional.” One crew member said the intercept was “unprofessional” because of “the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft.”

The WC-135 has been regularly deployed on missions in Northeast Asia, according to US officials. The “nuke sniffer” reconnaissance planes have been used in the past to gather evidence of nuclear activities by North Korea, according to reports by US media.

The US’s military presence in the region concerns China, which says such presence causes unnecessary tensions.

Chinese forces conducted two more interceptions last year, one in the South China Sea and one in the East China Sea. In the second case, a Chinese J-10 fighter came within 100 feet of a US Navy RC-135 reconnaissance aircraft.