A submarine that briefly intruded into Japanese waters last week was tracked by U.S. Navy P-3C patrol planes off Guam until it moved to waters near Okinawa, Japanese government sources said Tuesday.

Tokyo determined the sub to be Chinese and lodged a protest. On Tuesday, Beijing acknowledged it was a Chinese vessel and expressed regret over the incident.

Based on the positional information obtained by the P-3Cs, which tracked the nuclear-powered sub from the Guam area, Maritime Self-Defense Force patrol planes dropped a sonobuoy for detecting submerged submarines and confirmed the vessel's location south of Ishigaki Island in Okinawa Prefecture early Nov. 9, the sources said.