Chinese fighters intercepted an Air Force aircraft over the East China Sea Wednesday in an "unprofessional" manner, the Air Force said.

Two Su-30 fighters flew alarmingly close to a radiation-sniffing American WC-135 Constant Phoenix aircraft, Pacific Air Forces said in a statement Thursday. PACAF said the WC-135 was "conducting a routine mission in international airspace ... in accordance with international law."

The statement did not further detail the Chinese fighters' behavior, but an ABC report said one flew inverted over the Air Force plane and conducted a barrel roll, and both fighters came within 150 feet of the WC-135.

"While we are still investigating the incident, initial reports from the U.S. aircrew characterized the intercept as unprofessional," the statement said. "The issue is being addressed with China through appropriate diplomatic and military channels."

In a follow-up email Friday, PACAF spokeswoman Lt. Col. Lori Hodge said that the Air Force considered the encounter unprofessional "due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft," but declined to describe them further.

"Distances always have a bearing on how we characterize interactions," Hodge said. "Each interaction is different, and in this case, we are not providing more specifics as we would rather discuss it privately with China through our Military Maritime Consultative Agreement meeting and through diplomatic channels. This will allow us to continue building confidence with our Chinese counterparts on expected maneuvering to avoid mishaps."

Hodge also said that most military aircraft interactions, including most U.S.-China interactions, are safe and professional.

The Chinese interception follows a pattern of Russian planes flying dangerously close to U.S. aircraft over the last year. However, Gen. Tod Wolters, commander of U.S. Air Forces in Europe and U.S. Air Forces Africa said in March that the trend of Russian intercepts had "plateaued" over the previous six months.