Tokyo (CNN) -- Six Chinese maritime surveillance ships 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.
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, citing a government statement.
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 Chinese ships entered Japanese territorial waters despite warnings from the Japanese Coast Guard, said Shinichi Gega, a spokesman for Japan's 11th Regional Coast Guard Headquarters. Three of the ships have since left Japanese waters, he said.
The Japanese government will "take all possible measures to ensure security" around the islands, Prime Minister Yoshihiko Noda said Friday.
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 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."
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.
CNN's Junko Ogura in Tokyo and Jethro Mullen in Hong Kong contributed to this report.