Eight Chinese government ships have entered disputed waters apparently headed for the Senkaku-Diaoyu islands.
Japan has warned that it will "expel by force" any Chinese landing on the islands.
The latest clash over the islands came as nearly 170 Japanese lawmakers visited the controversial Yasukuni war shrine in central Tokyo on Tuesday, seen as a potent symbol of Japan's imperialist past, riling its neighbours China and South Korea.
Tokyo summoned the Chinese ambassador to Japan after the state-owned Chinese ships entered its territorial waters while Beijing called the shrine visit an "attempt to deny Japan's history of aggression".
The islands are surrounded by rich fishing grounds and are believed to harbour vast natural resources below the seabed.
Prime Minister Shinzo Abe vowed to "expel by force" any Chinese landing on the islands in the East China Sea, and promised "decisive action".
"We would never allow a landing," Mr Abe told parliament in response to questions from lawmakers, adding: "It would be natural for us to expel by force if [the Chinese] were to make a landing," he said.
It is feared in some quarters that war could inadvertently be triggered by Japanese and Chinese brinkmanship.