Two Chinese fighter jets intercepted a U.S. Air Force aircraft flying over the East China Sea on Wednesday, according to reports.

The U.S. military did not say whether the encounter was unsafe, but the crew of the aircraft reportedly called the interception “unprofessional.”

Reports said the U.S. WC-135 Constant Phoenix aircraft was flying in international airspace over the sea, when China’s Sukhoi Su-30 fighter jets intercepted it. The aircraft is known as a nuke sniffer and is designed to detect radiation.

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

"The issue is being addressed with China through appropriate diplomatic and military channels," Hodge said, according to Reuters.

According to Hodge, the description of the incident was based on initial reports from the U.S. aircraft crew "due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircrafts."

"Distances always have a bearing on how we characterize interactions," Hodge said.

An investigation into the incident has been launched by the U.S. military.

The Su-30 jets came within 150 feet of the WC-135 aircraft, a U.S. official told CNN, adding one of the Chinese aircrafts flew inverted, or upside down, directly above the American plane.

The U.S. military deployed the WC-135 to counter the growing nuclear threat from North Korea. The aircraft can keep a check on nuclear weapons testing and can track radioactive activity by collecting samples of particles and chemical substances.

The latest incident was the second bit of trouble this year involving a Chinese aircraft and a U.S. plane. On Feb. 8, a U.S. Navy P-3 spy plane and a Chinese military aircraft flew closely to each other over the South China Sea. At that time the U.S. called the incident unsafe.

China is reportedly suspicious of any U.S. activity in the region, mainly because of the territorial dispute over the South China Sea. Beijing has laid claims to almost all of the South China Sea, where Washington has been conducting navy patrols from time to time under freedom of navigation.

Last June, two Chinese J-10 fighter planes intercepted a U.S. Air Force RC-135 reconnaissance plane over the East China Sea. The maneuver, again, characterized as “unsafe” by the United States Pacific Command.

"One of the intercepting Chinese jets had an unsafe excessive rate of closure on the RC-135 aircraft. Initial assessment is that this seems to be a case of improper airmanship, as no other provocative or unsafe maneuvers occurred," the Pacific Command reportedly said at the time

China's Defense Ministry however, reportedly said the U.S. was just “hyping up the issue.”

"Judging by the report, the U.S. side is again deliberately hyping up the issue of the close surveillance of China by U.S. military aircraft," the ministry said then. "Chinese military pilots consistently carry out operations in accordance with the law and the rules, and are professional and responsible."