AFP news agency quoted a coastguard official as saying that even worse, the activists were flying the mainland flag. The report said they were ordered to leave Taiwan's waters.

The Hong Kong and Macau members of the Action Committee for Defending the Diaoyu islands set sail on Sunday in a fishing boat to assert China's sovereignty over the islands which are also claimed by Japan.

They were planning to meet up with activists from Taiwan but according to the Central News Agency, the Taiwanese activists agreed not to go after failing to hire a ship.

Meanwhile, the Chief Executive C Y Leung has expressed concern for the safety of the Hong Kong activists amid worsening weather. Tropical storm Kai-tak is heading towards Taiwan from the Philippines. Mr Leung said he hoped the activists would be able to get back safely.

Reports from Taipei said authorities had agreed to allow the activists' ship to sail to a safe port to take shelter from the storm.

The pro-China activists have made repeated attempts to reach the islands, but in the past they've been prevented from leaving Hong Kong by the government and apart from one successful foray in 1996 the group's ships have also been blocked by Japanese patrol vessels.