China has demanded that Japan immediately cease actions "harming" its territorial sovereignty amid an escalating dispute over rival claims to islands in the East China Sea.

China's foreign ministry made the comment in a statement reacting to plans by a group of Japanese to sail Saturday to the uninhabited islands after Tokyo deported pro-China activists who had sailed there from Hong Kong.

"China has made solemn representations to Japan, demanding that it immediately cease actions harming China's territorial sovereignty," the statement said.

The statement by Qin Gang, a ministry spokesperson, was in response to a media question regarding the planned trip to the contested islands by Japanese lawmakers and nationalist groups.

"China reiterates that any unilateral action taken by Japan regarding" the islands "are illegal and invalid", it said, adding that such actions will not undermine its claim over the territory.

It follows another statement late Friday that called on Japan to pursue "dialogue and negotiation" to resolve the dispute.

On Friday, a group of pro-China activists who sailed to a disputed islands in the East China Sea, known as Senkaku in Japan and Diaoyu in China and controlled by Japan, were deported.

Some of the activists had become the first non-Japanese to set foot on any part of the East China Sea archipelago since 2004.

Reinforcing coast guard

In Japan, the ruling party policy chief said on Saturday the country should strengthen its coast guard to defend the disputed islands.

"Coast guard officials are doing their best, and so the government and the ruling parties will discuss how to strengthen our backup to them," Seiji Maehara, the policy chief of the Democratic Party of Japan, told reporters.

"We should discuss not only [increasing] the number of staff and ships but also possibilities of various other supports" to the coast guards, he said.

In a separate statement, Taiwan accused Japan of "furtively occupying" the islands.

Last week, South Korean President Lee Myung-Bak paid an unprecedented visit to separate islands administered by Seoul and claimed by Japan.

In response, Japan recalled its ambassador from South Korea, cancelled its finance minister's planned trip to Seoul and called on South Korea to resolve the dispute in the International Court of Justice.