Chinese surveillance ship
Six Chinese surveillance ships entered waters near disputed islands claimed by Tokyo and Beijing on Friday, 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".
"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, reports Reuters.
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.
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 $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.