"We have confirmed some activists landed on Uotsurijima island," a local official of the Japan Coast Guard told AFP, referring to one of a chain of islands known as Diaoyu in China and Senkaku in Japan.

The official said he could not say immediately how many activists reached the island.

Public broadcaster NHK reported seven activists jumped into the water from their boat and swam to the rocky shore.

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 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 southeast China.

They said the move was aimed at countering a plan by a group of Japanese lawmakers to visit the disputed islands.

But the meet-up from Taiwan was aborted after four Taiwanese activists cancelled their voyage.

Wednesday was the 67th anniversary of Japan's surrender at the end of World War II. Tokyo is embroiled in an increasingly bitter spat with South Korea over another archipelago.

"The Japanese side has just fired water cannon at our boat," Action Committee for Defending the Diaoyu Islands chairman Chan Miu-tak told AFP in Hong Kong, after he spoke to one of the 14 activists on the boat.

The vessel was about 15 nautical miles from the disputed islands, known as Diaoyu in China and Senkaku in Japan, when the coastguard fired the water cannon, he said.

The coastguard warned the boat, which left Hong Kong on Sunday, not to enter Japanese waters before it resorted to using its water cannon.

Twelve Japanese ships were following the fishing boat and a helicopter was hovering around.

"I have told them safety comes first. We will retreat if there are any clashes," said Chan, who is not part of the team on the boat.

A Japanese coastguard official said the pro-China activists "entered Japanese territory at 15:51 (0651 GMT)".

"We are giving them a warning, using amplifiers and radio communications," he said.

But for "operational reasons" he declined to confirm whether the coastguard vessels had used water cannon.

He said the coastguard had dispatched patrol ships and helicopters but would not specify how many.

"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 Japan's coastguard.