Japan's Defense Ministry said Thursday 11 Chinese warships were spotted in international waters off the country's southern island of Okinawa.
No territorial violations were claimed by Japan, but the movements are sensitive because Japan and China have a dispute over small islands in the East China Sea.
The ministry on Thursday said the Chinese warships were monitored passing from the Pacific Ocean into the East China Sea.
Ministry spokesman Shuichi Fukuya said they were believed to be returning from target practice and refueling exercises in waters about 1,500 kilometers (930 miles) south of Okinawa.
He said the Japanese military saw the Chinese warships heading out to the area June 8-9.
Separately, the foreign ministry said a Chinese maritime research vessel briefly entered Japan's exclusive economic zone Thursday, around 330 kilometers (205 miles) off the coast of Miyagi, one of the worst-hit areas in the March 11 earthquake and tsunami.
A coast guard patrol vessel issued a warning against the Chinese ship, which left the economic zone around four hours later.