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May 19, 2017, 12:31 AM UTC / Updated May 19, 2017, 12:33 AM UTC

Two Chinese fighter jets intercepted a U.S. "sniffer" plane over the East China Sea, two U.S. military officials told NBC News on Thursday.

The WC-135 Constant Phoenix aircraft, which "sniffs" the atmosphere for signs of nuclear activity, was conducting a routine mission Wednesday in international airspace over the East China Sea when two Sukhoi Su-30 fighter aircraft intercepted it, the officials said.

The officials wouldn't say whether the confrontation was considered unsafe, but they said the crew of the U.S. plane described it as "unprofessional." The U.S. plane was operating in accordance with international law, they said.

Officials said the matter was being addressed with China through "appropriate diplomatic channels."