NBC News reported that the US aircraft, a WC-135 Constant Phoenix, was conducting a routine mission in international airspace on Wednesday when it was intercepted over the East China Sea by two Chinese Sukhoi Su-30 fighter aircraft.

The WC-135 is a so-called "sniffer plane" designed to scan the atmosphere for signs of nuclear activity.

NBC said the crew of the US plane described the encounter as "unprofessional," although not necessarily dangerous.

It said military officials insist the US plane was operating in accordance with international law.

Officials told the US news network that the matter is being addressed with Beijing via "appropriate diplomatic channels".