There has been a new upsurge of piracy off Somalia, reports an agency associated with the North Atlantic Treaty Organisation (Nato).

"Following a quiet period during late December, pirate activity increased significantly as the new year began," says the Nato Shipping Centre, the outreach arm of an agency set up to improve collaboration between military commanders and the commercial shipping industry.

News of the upsurge coincided with an announcement by the United States of a new multinational naval force to provide security in shipping lanes off the Somali coast.

The shipping centre said 15 vessels were currently in the hands of pirates, who were holding more than 200 merchant seafarers. It said the pirates now were targetting larger cargo ships and oil, gas or chemical tankers. They were also using more speedboats, each manned by three to five armed men.

The centre added that two vessels hijacked in the Gulf of Aden in the past 10 days, the MV Blue Star and the MV Sea Princess II, had been moved to the coast near Eyl, a port in the semi-autonomous region of Puntland which has become a pirates' haven.

As the vessels arrived there, the Turkish ship, Yasa Neslihan, was released after the payment of "an undisclosed ransom demand."

On Friday, the Associated Press reported that a negotiator for pirates holding the Sirius Star, a Saudi supertanker, had said the ship had been released after the pirates had been paid a ransom of U.S. $3 million.