Dozens of Taiwanese vessels, including coastguard ships, have entered territorial waters around disputed East China Sea islands, reports say.
The uninhabited islands, called Senkaku in Japan and Diaoyu in China, are claimed by Japan, China and Taiwan.
The Japanese government's recent purchase of the islands from a private owner has fuelled an escalating row.
According to Japan's Kyodo news agency, the Taiwanese ships are thought to be protesting against the nationalisation.
The boats entered the area early on Tuesday morning, accompanied by 10 Taiwanese surveillance ships, Kyodo quoted the Japanese coastguard as saying.
AFP news agency said that six Taiwanese coastguard vessels were among the boats.
The move comes as Japanese Vice-Foreign Minister Chikao Kawai arrives in Beijing for talks aimed at easing tension in recent weeks that saw massive anti-Japanese protests in China.