Story highlights All the Chinese ships have left the area, Japan says

Tokyo says it has protested the "illegal act" with Chinese authorities

Tensions between Japan and China are high over a group of disputed islands

Japan controls the islands, but China claims they are part of its territory

Six Chinese maritime surveillance ships briefly entered waters around a group of islands at the center of a heated territorial dispute between Tokyo and Beijing, ignoring warnings from the Japanese authorities amid escalating tensions in the region.

The Chinese ships arrived near the uninhabited islands -- which Japan calls Senkaku and China calls Diaoyu -- on Friday morning and began patrols and "law enforcement," China's state-run news agency Xinhua reported.

The islands, situated in the East China Sea between Okinawa and Taiwan, are currently under Japanese control, but China claims they have been an "inherent" part of its territory "since ancient times." The long-running argument over who has sovereignty has resulted in occasionally violent acts of public protest.

The United States,a key ally of Japan, has repeatedly urged Tokyo and Beijing to resolve the dispute through dialogue. U.S. Defense Secretary Leon Panetta will meet with his counterparts in Japan and China during a visit to the region that begins this weekend, the Department of Defense said Thursday

The Chinese ships entered Japanese territorial waters Friday despite warnings from the Japanese Coast Guard, said Shinichi Gega, a spokesman for Japan's 11th Regional Coast Guard Headquarters.

The vessels had all left the waters by mid-afternoon and headed north, the Japanese Coast Guard said later Friday, noting that sea in the area was getting rough as a huge storm, Super Typhoon Sanba, approached from the south.

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Japanese Chief Cabinet Secretary Osamu Fujimura said Japan would intensify its own patrols of the area in response to what he described as an "unprecedented scale of invasion" of Japanese waters.

Tokyo has protested the "inappropriate, illegal act" to the Chinese authorities, Fujimura said.

Prime Minister Yoshihiko Noda said Japan would "take all possible measures to ensure security" around the islands.

Two of the Chinese ships responded to a Japanese Coast Guard vessel's warning by reiterating China's territorial claim to the islands and saying they were carrying out patrol work, according to Gega. Japanese ships and helicopters are continuing their own patrols of the area, he said.

The controversial Chinese move to begin patrols around the islands follows the Japanese government's purchase of several of the islands from a private Japanese owner earlier this week, a deal that China described as "illegal and invalid."

The purpose of the patrols is "to demonstrate China's jurisdiction over the Diaoyu Islands and its affiliated islets and ensure the country's maritime interests," Xinhua reported Friday, citing a government statement.

This week, China announced what it said were the boundaries of its territorial waters around the islands to back up its claim of sovereignty. It said it had filed a copy of the announcement with U.N. Secretary General Ban Ki-moon on Thursday to comply with international law.

But Fujimura insisted Friday that the islands are an "integral part of Japanese territory" under international law, highlighting how directly opposed the two sides are.

Animosity between the two countries over the islands runs deep.

They have come to represent what many Chinese people see as unfinished business: redressing the impact of the Japanese occupation of large swathes of eastern China during the 1930s and 1940s.

China says its claim goes back hundreds of years. Japan says it saw no trace of Chinese control of the islands in an 1885 survey, so formally recognized them as Japanese sovereign territory in 1895.

Japan then sold the islands in 1932 to descendants of the original settlers. The Japanese surrender at the end of World War II in 1945 only served to cloud the issue further.

The islands were administered by the U.S. occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.

Tokyo's diplomatic corps suffered an unexpected setback Thursday when the newly appointed Japanese ambassador to China, Shinichi Nishimiya, collapsed in Tokyo and was hospitalized just two days after he was named to the post.