Kiyoshi Takenaka and Sui-Lee Wee -
Six Chinese surveillance ships entered waters near disputed islands claimed by Tokyo and Beijing on Friday, raising tensions between Asia's two biggest economies to their highest level since 2010 over a long-running territorial row.
Japan protested to China and urged that the situation not be allowed to escalate - an outcome neither side would welcome given the two countries' tight economic links.
Diplomats say Tokyo and Beijing would prefer to keep the row from spiralling out of control, but with China facing a once-in-a-decade leadership change, an election looming in Japan and mutual mistrust deep, managing the feud could be difficult.
"The dangers of miscalculation are real," said Brad Glosserman, executive director at Honolulu's Pacific Forum CSIS. China's foreign ministry said that the ships entered the disputed waters to conduct 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.
The coast guard said it ordered the Chinese ships to leave the area, but only three complied. No force had been used to expel the Chinese ships, a coast guard official said.
Chinese state television showed pictures of an official onboard a Chinese ship radioing a warning to Japanese ships around the island to withdraw from China's islands.