Four Chinese government ships entered territorial waters around disputed Tokyo-controlled islands on early Thursday, the Japanese coastguard said, sparking a strong protest from Japan.
Three maritime surveillance vessels entered the 12-nautical-mile zone around one of the islands in the East China Sea shortly after 6.30am (2130 GMT Wednesday), the Japanese coastguard said in a statement.
Another surveillance ship joined them one hour later.
The four Chinese vessels were off Minamikojima, one of the islands in a chain known as the Senkakus in Japan and the Diaoyus in China.
Separately, two fisheries patrol ships were spotted in the so-called contiguous waters, which extend a further 12 nautical miles, of another island in the chain, coastguards said.
The two vessel types are run by different Chinese government agencies but are not military.
Japanese vice foreign minister Chikao Kawai "strongly protested to the Chinese ambassador by telephone about the Chinese ships' intrusion into Japan's territorial waters," the foreign ministry in Tokyo said in a statement.
In Beijing, foreign ministry spokesman Hong Lei said there was nothing abnormal about Chinese ships exercising jurisdiction in the area.
"The Chinese maritime surveillance vessels conducted routine patrols in the territorial waters around China's Diaoyu Islands to safeguard the country's sovereignty on October 25," he said, according to the official Xinhua news agency.
Tensions have risen in recent months over the islands, which lie in rich fishing grounds. The seabed in the area is also believed to harbour mineral reserves.
After weeks of a sometimes bitter diplomatic stand-off over the issue, which has affected multi-billion dollar trade ties between Asia's two biggest economies, senior officials were reported to be readying for further talks.
Arrangements are being made for a meeting in Tokyo next week between Kawai and his Chinese opposite number, Zhang Zhijun, to discuss the island dispute, the Mainichi Shimbun said on Thursday.
The meeting would follow unannounced talks in Shanghai last weekend, the Japanese daily said.
The two officials also met in Beijing in September.