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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.

The submarine continued to travel submerged from the Guam area until MSDF planes and vessels stopped tracking it Friday in the East China Sea, they said.

It is highly probable that a submarine from the U.S. 7th Fleet was the first to detect the Chinese submarine in the Pacific after a U.S. military satellite system apparently detected a Chinese Han-class sub departing from a Chinese port, the sources said.

Defense Agency sources had earlier said the sub was thought to be a Han class — China’s first nuclear submarines, which went into service in the 1980s — based on noise analysis.

The sub moved through a corridor between Ishigaki and Miyako islands at around 5:50 a.m. Wednesday, breaching Japanese territorial waters for about two hours.

Since the 1990s, the Chinese Navy has been exploring submarine routes that will take vessels to the Pacific between Taiwan and Okinawa, the sources said.

China has designated the area connecting Tanegashima Island to Amami-Oshima Island, both in Kagoshima Prefecture, and the Sakishima island chain, which includes Ishigaki Island, as one route, and another route between Tokyo Bay and the Izu and Ogasawara island chains, with Guam at its periphery.

“I have heard that the Chinese submarine did not advance into the east side (toward the United States) of the second island chain line” linking Tokyo Bay to the island chains south of Tokyo, a government source said.

The intrusion prompted Defense Agency Director General Yoshinori Ono to mobilize the MSDF for enhanced maritime security at 8:45 a.m. Wednesday. Such an order has been issued only once before in the 50-year history of the Self-Defense Forces. That was in 1999, when two North Korean spy ships were chased away after nearing Ishikawa Prefecture’s Noto Peninsula.

The order was lifted at 3:50 p.m. Friday, and MSDF P-3C patrol planes, destroyers and helicopters stopped tracking the sub as it moved away from Japanese territory, judging there was little likelihood it would backtrack.

Military analyst Kenji Ebata said that while it was unclear how close the sub had been to Guam, he had never heard of Chinese subs operating so far away from the Chinese coast.

“It may have been training for encounters with U.S. (military) vessels,” he said, adding he was surprised the Chinese side would undertake such activities with Han-class subs, which are large and noisy.

Ebata also said he had no idea why the sub then entered Japanese territorial waters, noting that further information, including how long it took to travel from waters off Guam to those near Ishigaki Island, would be necessary before making such an analysis.

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