The United States cautioned China and Japan against escalating a row over a group of islands that both nations claim, warning that tensions between the world's second and third-biggest economies would have global repercussions.

On Tuesday, Japan brushed off stern warnings by China and said it had bought the islands from a private Japanese owner. China rained warnings on Japan and official media said Beijing had sent two patrol ships to reassert its claim.

"This is the cockpit of the global economy and the stakes could not be bigger and the desire is to have all leaders to keep that squarely in mind," U.S. Assistant Secretary of State Kurt Campbell said in Washington.

"We think in the current environment we want cooler heads to prevail, frankly," Campbell said while answering questions at a debate held by the Centre for Strategic and International Studies.

"We have enormous stakes in the maintenance of peace and stability. We believe that peaceful dialogue and the maintenance of peace and stability is of utmost importance always but particularly now in this set of circumstances."

Although a close ally of Japan, Washington has said it will not take sides in the territorial dispute.

The Chinese military's top newspaper accused Japan of "playing with fire", and the Ministry of Defence warned that more, unspecified steps could follow.

"The Chinese military expresses its staunch opposition and strong protest over this," Defence Ministry spokesman Geng Yansheng said in remarks posted on the ministry's website (www.mod.gov.cn).

"The Chinese government and military are unwavering in their determination and will to defend national territorial sovereignty. We are closely following developments, and reserve the power to adopt corresponding measures."

Tokyo insisted it had only peaceful intentions in making the 2.05 billion yen ($26.18 million) purchase of three uninhabited islands in the East China Sea, until now leased by the government from a Japanese family that has owned them since early 1970s.

Foreign Minister Koichiro Gemba repeated Japan's line that the purchase served "peaceful and stable maintenance of the islands".

"We cannot damage the stable development of the Japan-China relationship because of that issue. Both nations need to act calmly and from a broad perspective," he told reporters.

The Japanese Coast Guard will administer the islands, called Senkaku in Japan and Diaoyu in China, which are near rich fishing grounds and potentially huge maritime gas fields.

Geng accused Japan of "using all kinds of excuses to expand its armaments, and repeatedly creating regional tensions".

Beijing has avoided sending military forces into disputed seas at the heart of quarrels with neighbors, including Japan, instead using civilian government vessels to stake its claims.

China's Xinhua news agency reported that two China Marine Surveillance (CMS) vessels reached the waters around the islets on Tuesday morning. The government force is in charge of enforcing law and order in China's waters, but operates separately from the navy.

CHINESE ANGER

The tensions with Japan come while China's ruling Communist Party is preoccupied with a forthcoming once-in-a-decade leadership change, as well as slowing economic growth.

China's focus on domestic politics and the economy will not deter a potentially strong response to Japan, said Sun Cheng, a professor specializing in Japan at the China University of Political Science and Law in Beijing.

"Chinese people won't disregard territorial disputes just because of the economy and the Party Congress," said Sun. "And if China is too soft on this issue, I don't think the Chinese people will abide by that."

The news triggered small-scale protests in front of the tightly guarded Japanese embassy in Beijing. Microbloggers on China's popular Twitter-like service Sina Weibo also reported small anti-Japanese protests in the eastern city of Weihai and the southwestern city of Chongqing.

"We strongly urge Japan to fully grasp the danger of the present situation and step back from the edge of a precipice over the Diaoyu islands issue," the foreign affairs committee of China's national parliament said in a statement read out on a state television news broadcast.

The long-running territorial dispute flared last month after Japan detained a group of Chinese activists who had landed on the islands. And the row appears to be having an economic impact, with a Chinese official saying Japanese car sales in the world's biggest auto market may have been hit.

Japanese Prime Minister Yoshihiko Noda, in an address to senior military officers, made no direct reference to the islands dispute, but pointed to China's growing military clout as one of challenges Japan had to contend with.

The Japanese foreign ministry said it was sending its Asia department chief to Beijing on Tuesday for talks to "avoid misunderstanding and lack of explanation on the issue".

The government bought three of five islets that it has been leasing from the Kurihara family, which bought the islands in 1972 from another Japanese family that had controlled them since the 1890s. The government has owned one of the remaining islets and continues to lease one from the Kurihara family.

Noda floated the plan to buy the islets in July to head off what appeared to be a much more provocative bid by Tokyo governor Shintaro Ishihara, a harsh critic of China, to purchase them and make the islands available for development.