A boat with seven Hong Kong and Taiwanese activists on board was Thursday heading for islands at the centre of a bitter international wrangle, a report and Japan's coastguard said.
By around 7:00 am (2200 GMT Wednesday) the vessel was 107 kilometres (66 miles) away and "sailing towards
the Senkakus", Japan's coastguard said, when asked about a report from Jiji of a vessel that left Taiwan earlier.
China and Taiwan both claim the island chain, which is controlled by Tokyo.
The uninhabited islands, whose seabed is believed to harbour valuable mineral reserves, has been the scene of a diplomatic tussle between Japan and China over the past few months.
Since Tokyo nationalised three of them in September, Beijing has repeatedly sent government ships into their waters. In December a government plane overflew them, leading Japan to scramble fighter jets.
Earlier this month both militaries had jets in the area and Japanese newspapers have reported that Tokyo is mulling allowing its pilots to fire warning shots.
While most commentators believe Asia's two largest economies will find some way to work around the problem, which has rattled relations for decades, some are warning that a mis-step could lead to armed confrontation.