Japan protested Monday after two Chinese survey ships entered what Tokyo considers its waters, reigniting a long-simmering territorial row between the two Asian giants.

"Such activities by Chinese ships are extremely regrettable. We are demanding the Chinese government order their immediate withdrawal," top government spokesman Takeo Kawamura told reporters.

The two ships were spotted in the morning by the Japanese coast guard six kilometres (four miles) southeast of the uninhabited Senkaku, or Diaoyu islands, which are claimed by Japan, China and Taiwan.

"The Senkaku islands are an integral part of our traditional territory," said Kawamura, describing the incident as "unacceptable."

A Japanese patrol vessel issued verbal warnings to the two ships, which both belong to the Chinese government, a coast guard official said. Later, the coast guard confirmed the two vessels left the area in the East China Sea.

Prime Minister Taro Aso said the incident was "an obvious intrusion of (Japan's) territorial waters."

Tokyo lodged a formal protest to Beijing over the intrusion, the first time since February 2007 that Japan has spotted Chinese survey ships near the disputed islands.

"Japan immediately communicated a protest to China's foreign ministry through diplomatic channels, and strongly requested that (the ships) promptly leave the territorial waters," said a foreign ministry spokeswoman.

Japanese envoy Akitaka Saiki delivered the protest to his Chinese counterpart Wu Dawei in Beijing ahead of six-nation talks on the North Korean nuclear crisis, officials said.

There was no immediate response from Beijing.

Japan has made similar protests in the past over maritime research by China in what Tokyo considers to be its waters, but the two countries have been working to improve ties recently.

The latest spat comes as Aso prepares to host a summit with Chinese Premier Wen Jiabao and South Korean President Lee Myung-Bak in the western Japanese city of Fukuoka on Saturday.

It will be the first such summit to be held separately from multilateral forums.

Japan declared the islands part of its territory in 1895 when it took over Taiwan. After World War II, the United States used the islets for military drills before handing them to Japan in 1972 along with Okinawa.

China and Taiwan renewed their claims to the area after oil deposits were confirmed nearby in the 1970s.

Tokyo and Beijing have been trying to repair relations, which were badly strained under Japan's former prime minister Junichiro Koizumi over memories of Japan's aggression in Asia before and during World War II.

Japan and China were also embroiled in a long-running row over the rights to lucrative gas resources in the East China Sea, but struck a landmark deal in June to jointly develop the fields.