The sentiment of protesters outside the Japanese embassy in Beijing on Tuesday is aptly summed up by the message on a piece of paper: "The Diaoyu Islands belong to China, Japanese get out." [Photo/Agencies]

The military said on Tuesday it "reserves the right" to take action on the Diaoyu Islands after the Japanese government ignored warnings from Beijing and "purchased" three of the islands, which belong to China.

Two China Marine Surveillance patrol ships reached waters around the islands, in the East China Sea, after Beijing announced on Monday territorial coordinates for waters off the islands. Beijing also announced plans to implement normal surveillance and monitoring of the islands.

The Japanese government on Tuesday "purchased" the three islands for 2.05 billion yen ($26.18 million) from a "private owner".

The deal was signed shortly after the Japanese cabinet decided to use reserve funds for the "purchase".

Vice-Premier Li Keqiang told visiting Papua New Guinea Prime Minister Peter O'Neill on Tuesday that the Japanese government's deal to "purchase" the Diaoyu Islands from the so-called owner severely undermines China's territorial sovereignty.

Shinsuke Sugiyama, director-general of the Asian and Oceania Affairs Bureau at the Japanese Foreign Ministry, arrived in Beijing on Tuesday afternoon for urgent talks to "avoid misunderstanding and lack of explanation on the issue''.

But Chinese experts said the move proves that Tokyo has shown no sincerity in finding a resolution.

Sugiyama's trip is an attempt to justify the "purchase" deal and "has not, in the least, shown Tokyo's sincerity" for a resolution, said Lu Yaodong, director of the department of Japanese diplomacy of the Institute of Japanese Studies at the Chinese Academy of Social Sciences.

"Sugiyama's trip is aimed at expressing Japan's national interests while Tokyo has shown little consideration for the shared interests of both sides," Lu said.

China-Japan ties have been strained since Shintaro Ishihara, the right-wing Tokyo governor, unveiled plans on behalf of the city government to "buy" the islands in April.

Japanese Prime Minister Yoshihiko Noda announced a plan in July to "nationalize" the islands, a move that prompted immediate protests from Beijing.

Defense Ministry spokesman Geng Yansheng said on Tuesday that "we are closely watching the development of the situation, and we reserve the right to take countermeasures".

The Diaoyu Islands have been part of China's territory for centuries and this is supported by historical fact and jurisprudential evidence. The Japanese government's "purchase" of the islands is totally illegal and invalid, Geng said.

The foreign affairs committees of National People's Congress and the Chinese People's Political Consultative Conference also issued separate statements on Tuesday condemning the "purchase".

Two China Marine Surveillance patrol ships reached waters around the islands on Tuesday morning.

The CMS said it had drafted an action plan for safeguarding territorial sovereignty and would take action pending the development of the situation.

Noda had previously told the Japanese parliament that Tokyo cannot rule out the possibility of dispatching Japan's Self-Defense Force troops if the situation "gets out of control".

Given China's territorial definition, through the coordinates, entry into waters around the islands by the Japanese Coast Guard or Japan's Self-Defense Force troops will be regarded as an intrusion into China's territorial waters, said Feng Wei, a specialist on Japanese studies at Fudan University in Shanghai.

"And it is the duty and obligation of Chinese government vessels, and even warships, to guard China's territorial sovereignty," Feng said.

China also launched the first marine forecast service on Tuesday for the islands and surrounding waters through China Central Television.

The State Oceanic Administration said in a statement that the service is significant for safeguarding China's maritime rights and interests as well as protecting the safety of fishermen, fishing boats and ocean patrol ships in the area.

The administration will issue daily forecasts on ocean conditions to be broadcast via the national broadcaster, radio and Internet media.

Also on Tuesday, Taiwan recalled its representative to Japan in protest against the deal. A day earlier, Foreign Minister Yang Jiechi summoned, and made representations to, Japanese Ambassador to China Uichiro Niwa.

On Monday, Japan replaced Niwa with Shinichi Nishimiya, the first ambassador to China from the private sector.

The island "purchase" triggered small-scale protests in front of the tightly-guarded Japanese embassy in Beijing.

There were also small anti-Japanese protests in Guangzhou, capital of Guangdong province and Weihai, Shandong province as well as the southwestern city of Chongqing.

Reaction to the deal also swamped the Internet in China with nearly 13 million netizens following or commenting on related posts via Sina Weibo.

More than one million netizens have vowed on Sina Weibo to boycott Japanese goods.

Trade between the two countries is worth more than $340 billion.

"This means China has the cards in hand to take countermeasures in the economic field, especially against Japanese financial groups which support right-wing activities," said Ruan Zongze, deputy director of the China Institute of International Studies.

Sales of Japanese automobiles have fallen in China recently, partly because of the dispute over the islands. Some Chinese experts believe such repercussions could spread to other industrial sectors, like cell phones, personal computers and home appliances.

Shanghai media reported on Tuesday that 20 percent of the city's residents who had arranged trips to Japan have canceled.

Japan's Kyodo News Agency also said a meeting between the governor of Japan's Fukushima Prefecture and the chief of China's Civil Aviation Administration, scheduled for Tuesday, was canceled.

Vice-governor of Shandong province Xia Geng also canceled a business trip to Japan.

US State Department spokeswoman Victoria Nuland told reporters on Monday that Washington is concerned about rising tension between China and Japan, saying good relations between the two are crucial for the region and the United States.

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Reuters and Xinhua contributed to this story.