This happened 20 miles SE of Kinmen Island (old name, Quemoy). Kinmen Island is just off the mainland coast near Xiamen, but is controlled by Taiwan. 20 miles to the SE of the Island is well within international waters, in the Formosa Strait. Assuming the Chinese warship captain knew he was not in the very narrow passage between Kinmen Island and the port of Xiamen (which would be visually obvious and obvious on radar), this was basically an attempt at piracy.