Chinese fighter jets intercepted a U.S. aircraft as it was flying international airspace over the East China Sea on May 19.
The U.S. military called the interception “unprofessional.”
The “sniffer” plane was meant to detect radiation and has been used in the past to detect evidence of possible nuclear tests by North Korea.
The American aircraft was carrying out a routine mission and operating in line with international law, said a military spokesman.
Beijing has not commented on the incident.