Japan's coastguard stepped up security Wednesday in waters near disputed islands in the East China Sea as a group of pro-Beijing activists sailed towards the chain.
"We are on high alert so as to prevent them from entering Japanese territorial waters or landing on the islands, while exercising caution to avoid a possible collision," said a local official of the Japan Coast Guard.
A group of 14 pro-China activists from Hong Kong and Macau set sail on a Chinese-flagged fishing boat from Hong Kong on Sunday, heading towards the disputed islands.
The activists, who belong to a group called the Action Committee for Defending the Diaoyu Islands, had said they would be joined at sea by two other vessels, one each from Taiwan and Xiamen city in southern China.
They said the move was aimed at countering a plan by a group of Japanese lawmakers to visit the disputed islands, known as Diaoyu in China but controlled by Tokyo, which calls them Senkaku.
But the meet-up from Taiwan was aborted after four Taiwanese activists cancelled their voyage.
The activists were due to reach the islands on Wednesday, the 67th anniversary of Japan's surrender at the end of World War II and as Tokyo is embroiled in an increasingly bitter spat with South Korea over another archipelago.
"Our boat is about 50 nautical miles from the Diaoyu islands," the Action Committee for Defending the Diaoyu Islands chairman Chan Miu-tak told AFP in Hong Kong.
"Two Japanese coastguard ships are following us closely," said Chan, who was not part of the 14-member team.
The Japanese coastguard official said he could not confirm the claim.
"I haven't received a report that the activists have crossed the median line," the territorial border claimed by Japan, he said. China does not recognise the border.
The pro-China group has made repeated attempts to reach the islands, but apart from one successful foray in 1996 they have been blocked by Japanese patrol vessels.
The uninhabited outcrops were the scene of a particularly nasty confrontation in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of its coastguard vessels.
Tensions spiked in April after controversial Tokyo Governor Shintaro Ishihara said his city intended to buy the islands from their private Japanese owner.