Image copyright Reuters Image caption Tension over the disputed islands has even hit trade relations

China says six boats sent to "patrol" disputed islands at the centre of a row with Japan were successful in demonstrating Beijing's jurisdiction.

The statement was issued after anti-Japan protests spread to dozens of Chinese cities.

China sent the boats to the islands, called Diaoyu in China and Senkaku in Japan, after Japan sealed a deal to buy three of the islands.

US Defence Secretary Leon Panetta has called for restraint on both sides.

"A misjudgement on one side or the other could result in violence, and could result in conflict," said Mr Panetta, who is in Tokyo for talks but due to travel to Beijing this week.

Mr Panetta met his counterpart, Satoshi Morimoto, and Foreign Minister Koichiro Gemba.

"I did not bring up the topic today, but it is mutually understood between Japan and the United States that (the islands) are covered by the treaty," Mr Gemba told reporters after the meeting.

Mr Panetta also announced that US and Japanese officials had agreed to deploy a second missile defence system in Japan, designed to protect the region from the threat posed by North Korea.

'Remain calm'

Analysis Leon Panetta arrived in Japan for a short, but timely, visit. The mounting tension between his hosts and China is expected to dominate his meetings with the foreign and defence ministers. Panetta will travel next to China, where he may be the last high-ranking US official to visit before the country's once-a-decade leadership change. Ultimately, who is right on the territorial dispute? As far as the Obama administration is concerned, it's not about who owns the islands - it's about avoiding a costly action/reaction scenario that could lead to conflict between the two countries. As US Secretary of State Hillary Clinton said on her recent trip to the region, and Mr Panetta has reiterated, the US doesn't want to get involved in other nations' territorial disputes. However, while the relationship between the world's second and third largest economies, China and Japan respectively, tends to be somewhat strained - genuine unrest would be an international problem.

China, meanwhile, is maintaining its sovereignty over the uninhabited but resource-rich islands in the East China Sea which are controlled by Japan and also claimed by Taiwan.

The China Marine Surveillance (CMS) - which sent the boats on Friday - will continue with the ''law enforcement activities'' around the disputed islands, Chinese state media reported.

''These law enforcement and patrol activities demonstrated China's jurisdiction over the Diaoyu Island and its affiliated islets, achieved the goal of demonstrating China's sovereignty claim and ensured the country's maritime interests," Xiao Huiwu, deputy chief of the headquarters of CMS, told Xinhua news agency.

Anti-Japan protests swept across China over the weekend with thousands of protesters targeting Japanese-made cars and burning Japanese flags.

On Sunday, hundreds of Chinese protesters were met by riot police at the Japanese embassy in Beijing.

Japanese Prime Minister Yoshihiko Noda has urged Beijing to take steps to protect Japanese nationals, adding that the authorities in both countries should "remain calm".

The tension between the two countries has also hit trade relations and sparked off fears of wider economic consequences.

Japanese electronics giant Panasonic is suspending operations at its factory in Qingdao until Tuesday, after the plant was damaged by protesters on Saturday. Canon will also stop operations at three of its four plants in China on Monday and Tuesday, reports said.

Japan-China disputed islands The archipelago consists of five islands and three reefs

Japan, China and Taiwan claim them; they are controlled by Japan and form part of Okinawa prefecture

The Japanese government signed a deal in September 2012 to purchase three islands from Japanese businessman Kunioki Kurihara, who used to rent them out to the Japanese state

The islands were the focus of a major diplomatic row between Japan and China in 2010 Q&A: China-Japan islands row

Damage limitation

The islands have long been a point of contention, but the most recent row has heightened tension and led to fears of a naval conflict.

But analysts see Japan's decision to buy the islands as damage limitation in response to a much more provocative plan by the nationalistic governor of Tokyo, who wanted to purchase and develop them.

In Japan, the government seems unlikely to back down with an election looming.

There is even talk in the Japanese press that some on both sides could be willing to risk a limited naval clash in defence of the conflicting claims, correspondents say.

Further complicating matters, Japan's newly appointed ambassador to China, 60-year-old Shinichi Nishimiya, died on Sunday, the foreign ministry said.

The envoy - who had been due to take up his post in October - collapsed several days earlier near his Tokyo home and was taken to hospital.

The foreign ministry has not publicly commented on what caused his death.