Two Chinese navy frigate ships sailed through a zone just outside Japanese territorial waters between Yonaguni and Iriomote islands in Okinawa Prefecture on Tuesday, the latest in a series of increased Chinese maritime activities in recent days.
When passing through the area from 5 a.m. to 9 a.m. while en route from the Pacific Ocean to the East China Sea, the frigates headed for the Senkaku Islands at one point but changed course, Japan's Defense Ministry said.
The same ships were spotted Thursday around the area off Okinawa, according to the ministry.
The ships were again sailing in the contiguous zone outside Japanese waters on Tuesday despite Japan the previous day lodging a protest with China over a series of intrusions by Chinese ships into territorial waters around the Japan-administered Senkaku Islands, which China claims and calls the Diaoyu.
Japan has been on alert as China continues to send patrol ships and airplanes into the area in apparent protest against Japan's purchase in September last year of some of the islands from a private owner.
The Air Self-Defense Force scrambled fighter jets on Sunday to respond to four Chinese military aircraft flying over international waters between Okinawa's main island and Miyako Island.
Conflicting territorial claims over the islands have been one of the bottlenecks for the two countries to hold summit talks. Prime Minister Shinzo Abe has said Tokyo will not make concessions regarding sovereignty over the islets, while leaving the door open for dialogue.