Japan's prime minister says Tokyo will respond with "force" if China lands on islands at the center of a dispute between the two countries.
Shinzo Abe's comments on April 23 came after eight state-owned Chinese ships sailed near East China Sea islands that both countries claim.
In response, Tokyo summoned the Chinese ambassador to Japan.
The warning from Abe is the most explicit warning to China since he took power in December.
A flotilla of 10 fishing boats carrying Japanese activists was also reported to be in the area, as well as the Japanese coast guard.
The latest spike in tensions comes after 170 Japanese lawmakers on April 23 visited the controversial Yasukuni war shrine in Tokyo, a symbol of Japan's imperialist past.
That visit has angered China, which called it an "attempt to deny Japan's history of aggression."
Based on reporting by AFP and BBC