The Japanese government and coast guard said six Chinese surveillance ships entered Japanese waters on Friday near disputed islands in the East China Sea, adding to tensions between the Asian giants.

It was the first intrusion by Chinese vessels into what Japan says are its waters since Tokyo bought the islands from their private Japanese owners this week. The islands, claimed by both countries and called Senkaku by Japan and Diaoyu by China, are surrounding by rich fishing grounds and are near key shipping lanes.

After Japanese coast guard ships telegraphed warnings to the Chinese vessels, two or three moved beyond territorial waters but the others remained within the 12 nautical mile zone around the uninhabited islands that are controlled by Japan, said Yasuhiko Oku, an official with the Japanese coast guard in Tokyo.

One of the Chinese patrol vessel responded to the coast guard's warnings by demanding the Japanese ships get out of the waters immediately.

"Diaoyu is China's territory. This ship is carrying out lawful operations. We urge you to leave the waters immediately," Japan's coast guard quoted a

Chinese vessel as saying in radio communications.

Japanese chief cabinet secretary Osamu Fujimura said the territorial violation by a fleet of six Chinese vessels at once is "unprecedented." He said Japan strongly protested the intrusion and is staying on alert.

"It is extremely regrettable," Fujimura said at a regular news conference. "We strongly protest the intrusion by the latest Chinese vessels."

"We are taking all possible measures to be ready for any development," he said but refused to give details.

Anti-Japanese protests have been held in various Chinese cities, and some Chinese have called for a boycott of Japanese goods. Fujimura urged Japanese visitors and residents in China to take precautions. Japan also demanded Beijing to take steps to reassure the safety of Japanese tourists and residents in China.

The Chinese foreign ministry confirmed that six surveillance ships had entered waters near the islands, which are also claimed by Taiwan, and began "patrol and law enforcement" there.

"These law enforcement and patrol activities are aimed to demonstrate China's jurisdiction over the Diaoyu Islands and its affiliated islets and ensure the country's maritime interests," the statement said.

Japan's vice foreign minister Chikao Kawai summoned Chinese Ambassador to Japan Ting Yonghua to the ministry and lodged a protest.

Tensions between the two nations have flared since Tokyo's nationalistic governor, Shintaro Ishihara, in April proposed buying and developing the islands so that they wouldn't fall into Chinese hands. Activists from both sides landed on the islands in August.

To block Ishihara's plan, which would likely infuriate China, Prime Minister Yoshihiko Noda's government was left with little choice but to buy the islands.

The government doesn't plan to develop them, but the move still angered China, which warned of "serious consequences."

On Thursday, two Taiwanese coast guard patrol ships sailed to 25 nautical miles west of the disputed islands. The Taiwanese coast guard said the vessels demonstrated the coast guard's ability to protect local fishermen.