Six Chinese surveillance ships entered waters near disputed islands claimed by Tokyo and Beijing, raising the stakes in a long-running territorial row between Asia's two biggest economies.
China's foreign ministry said that the ships entered the disputed waters to carry out maritime surveillance and that for the first time China was carrying out a mission of "law enforcement over its maritime rights", on Friday (local time).
"It reflects our government's jurisdiction over the Diaoyu islands," it said in a statement. The ministry has used similar language in the past.
The islands, known as Senkaku in Japanese and Diaoyu in Chinese, are near potentially huge maritime gas and oil fields.
The uninhabited islets were at the centre of a chill in 2010 after Japan arrested a Chinese trawler captain whose boat collided with Japanese Coast Guard vessels near the area.
The Japanese coast guard said it ordered the ships to leave Japan's territorial waters, but only two complied, leaving four Chinese vessels still in the disputed area.
No force had been used to remove the Chinese ships, a coast guard official said.
"We'll do our utmost in vigilance and surveillance," said Prime Minister Yoshihiko Noda when asked about Japan's responses.
TRADE AT RISK
China warned Japan on Thursday that trade could be hurt by the flare-up in tension. China, the world's second-largest economy is Japan's biggest trading partner with mutual trade in 2011 growing 14.3 percent in value to a record US$345 billion.
A Nissan Motor Co Ltd executive has said the tensions were already affecting business with China.
Tensions flared last month when Japan detained a group of Chinese activists who had landed on the islands and Japanese nationalists landed on the islands. Anti-Japanese protests rocked several Chinese cities.
Bilateral relations were frayed further on Tuesday when Japan, which controls the islands, said it had bought them from a private owner, ignoring warnings from China.
China said this was a breach of its sovereignty.
China's official Xinhua news agency on Thursday said a senior Chinese military official had urged the army to be "prepared for any possible military combat", though the report made no mention of the territorial dispute with Japan.
"Efforts should be made to ensure that the military is capable of resolutely performing its duty to safeguard the country's national sovereignty, security and territorial integrity whenever it is needed by the Party and the people," Xu Caihou, vice chairman of China's powerful Central Military Commission, was cited as telling soldiers.
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.
Japan's ruling Democrats also face an election, probably late this year, which opinion polls suggest they will lose and the government is under fire for its handling of territorial rows with China, and with South Korea over another island chain.
The United States this week urged both sides to tone down increasingly impassioned exchanges over the longstanding row.
The last time Chinese government-affiliated ships entered Japanese territorial waters near the disputed islands was in mid-July, the coast guard said. That incident ended peacefully and without any significant diplomatic fallout.
The Japanese coast guard described those vessels as fishery patrol ships. The latest ships are civilian government vessels maintained by the State Oceanic Administration to patrol waters claimed by China.