Chinese patrol vessels have neared a disputed island group, prompting a warning from Japan's prime minister that Tokyo would "expel by force" any Chinese landing. Japanese nationalists also approached the islands.
"It would be natural for us to expel by force if [the Chinese] were to make a landing," Japanese Prime Minister Shinzo Abe told parliament on Tuesday.
The dispute centers on the archipelago known as Senkaku in Japan and Diaoyu in China and coincides with a fresh Chinese diplomatic protest over a visit by Japanese parliamentarians to a controversial war shrine in central Tokyo on Tuesday.
The Japan Coast Guard said eight Chinese patrol vessels entered the 12-nautical-mile zone around the islands. Japanese nationalists said nine of their boats had gone to the maritime area, which is also claimed by Taiwan.
Japan's Coast Guard said it had escorted the vessels away in an apparent attempt to avoid a direct clash.
The uninhabited islands are near rich fishing grounds and potentially lucrative maritime gas fields. Japan declared part of the archipelago nationalized last September.
Japan, China reciprocate with protests
Tokyo summoned China's ambassador to protest the patrol vessels' presence near the islands.
"It is extremely deplorable and unacceptable that Chinese government ships are repeatedly entering Japanese territorial waters," said Japan's Chief Cabinet Secretary Yoshihide Suga, referring to frequent sailings in the area by Chinese ships.
In turn, Beijing described the visit on Tuesday by 170 Japanese lawmakers to the Yasukuni Shrine in Tokyo as an "attempt to deny Japan's history of aggression" during the first half of the 20th century.
The shrine honors war dead including 14 former imperial Japanese leaders convicted after World War Two as war criminals by an Allied tribunal.
Abe and Deputy Prime Minister Taro Aso had visited Yasukuni over the weekend. A leading Liberal Democrat Party lawmaker Hidehisa Otsuji said he did not understand the backlash.
"It is natural for lawmakers to worship at a shrine for people who died for the nation and every nation does this," he said.
"No matter in what capacity or form Japanese leaders visit Yasukuni Shrine, in essence it is an attempt to deny Japan's history of aggression through militarism," said Chinese Foreign Ministry spokeswoman Hua Chunying.
In further reaction to the Yasukuni visits, South Korea, which has bitter memories of Japan's 1910-1945 colonization, also cancelled a scheduled trip to Japan by its foreign minister Yun Byung-Se.
A South Korean foreign ministry spokesman said matters of history should be kept separate from efforts to cooperate over North Korea's nuclear ambitions.
ipj/ccp (AFP, Reuters, AP)