HONG KONG (AFP) - Hong Kong activists left on Wednesday on what they called a "fishing" boat trip to disputed islands, more than a year after they sailed to another contested chain to assert China's sovereignty.

"We are going fishing," Mr Tsang Kin-shing told AFP aboard the fishing boat Kai Fung No. 2.

He refused to reveal the trip's itinerary, saying only that the group of 13, along with two journalists, would sail to the Spratly Islands, whose ownership is disputed between China and several other countries.

"If there are no fish at Nansha, then we will go anywhere within Chinese territory where there is fish, so we can't say right now where the most amount of fish are," said Mr Lo Chau, another activist.

The Spratlys in the South China Sea - known as the Nansha Islands in China - are disputed between Taiwan, Brunei, China, Malaysia, the Philippines and Vietnam.

In August last year the same group of nationalist activists used the boat to land on disputed islands in the East China Sea known as the Senkakus in Japan and as the Diaoyus in China.

They planted a Chinese flag but were arrested and deported by Japanese authorities, who control the islands.

Plans to repeat the voyage to the Diaoyus were foiled in August this year when Hong Kong's Marine Department stopped the 45m vessel from setting sail for "safety reasons".

Officials in the semi-autonomous southern Chinese city have several times tried to thwart trips by the activists, prompting speculation that Beijing does not favour such high-profile protests.

The group's departure from Hong Kong harbour was delayed on Wednesday morning because of inspections by police and other officials.

On one occasion, police officers wearing helmets and body armour boarded the boat and prevented it from setting sail for nearly an hour.

When the Kai Fung No. 2 finally set off, it was followed by several government vessels.

Mr Tsang questioned the motives for the official checks. "Do they have to prevent fishermen from going out to fish?" he said.