Somali pirates have hijacked a cargo ship off the Gulf of Aden, defying security patrols by several warships in the region, a Kenyan maritime official said on Tuesday, according to AFP.

The Philippine-flagged Stolt Strength, managed by a Panama company, was seized on Monday, said Andrew Mwangura who runs the Kenya chapter of the Seafarers Assistance Programme.

"The ship was seized on Monday afternoon, but we are now tracking its whereabouts," he told AFP.

"We are yet to establish where the ship was heading and what it was carrying," he added.

Stolt Strength, with 21 Filipino crew, was seized nearly two months after its sister freighter, Stolt Valor, was taken in the same region.

There have been least 81 pirates attacks in the region this year, including 32 hijackings, according to the International Maritime Bureau`s (IMB) piracy reporting centre in Kuala Lumpur.

Of that figure, 11 ships and more than 200 crew are still being held by Somali pirates.

On Monday, the European Union launched a security operation off the coast of Somalia -- its first-ever naval mission -- to combat growing acts of piracy and protect ships carrying aid agency deliveries.

Dubbed Operation Atalanta, the mission, endorsed by the bloc`s defence ministers at talks in Brussels, will be led by Britain, with its headquarters in Northwood, near London.

Pirates are well-organised in the area where Somalia`s northeastern tip juts into the Indian Ocean, preying on a key maritime route leading to the Suez Canal through which an estimated 30 percent of the world`s oil transits.

They operate high-powered speedboats and are heavily armed, sometimes holding ships for weeks until they are released for large ransoms paid by governments or owners.

NATO warships, along with ships and aircraft from several other nations have been deployed in the region to protect commercial shipping.