China has lodged "stern representations" with the United States, accusing it of sending a US U-2 reconnaissance plane into a no-fly zone over Chinese live-fire military drills, further ratcheting up tensions between Beijing and Washington.
China has long denounced US surveillance activities, while the United States has complained of "unsafe" intercepts by Chinese aircraft. While such missions happen regularly, for China to talk about them publicly is unusual.
China's defence ministry said the U-2 flew without permission over a no-fly zone in the northern military region where live fire drills were taking place, "seriously interfering in normal exercise activities".
This could easily have caused a misunderstanding or misjudgment or an "unexpected incident", the ministry said.
"It was an act of naked provocation, and China is resolutely opposed to it, and have already lodged stern representations with the US side."
In a statement, the US military said a U-2 flight was conducted in the Indo-Pacific region and it was "within the accepted international rules and regulations governing aircraft flights".
"Pacific Air Forces personnel will continue to fly and operate anywhere international law allows, at the time and tempo of our choosing," the US military said in the statement.
While China did not say exactly where the incident took place, it is carrying out drills in the Bohai Sea. Other exercises are also happening in the Yellow Sea and South China Sea.
Relations between Washington and Beijing have worsened over everything from trade and human rights to what the United States sees as aggressive moves by China's armed forces, especially in the disputed South China Sea and around Chinese-claimed Taiwan.