Their most audacious and significant attack to date came on Saturday with the capture of the Saudi-owned Sirius Star and its crew of 25 (including two Britons) which was taken 450 nautical miles (830km) off the coast of Kenya.

The move was described as "unprecedented" by the US Navy due to the size of the vessel and the distance of the attack from the pirates’ usual theatre of operations; the Sirius Star is the biggest ship taken by pirates so far, weighing three times more than an American aircraft carrier and capable of carrying 2m barrels of oil, valued at around $100m (683m Yuan).

The pirates, who are often armed with AK-47s and rocket-propelled grenades, usually hijack ships in the Gulf of Aden using powerful speedboats to approach merchant ships which they board using grappling hooks and rope ladders.

Once the crew has been taken hostage, the pirates direct the ship to a port in Somalia where the crew and boat are held until a ransom is paid. There are currently 14 ships and 268 crew being held in pirate-friendly Somali ports.

Given that senior UN officials estimate that the pirates earn more than $100m (683m Yuan) a year from ransom payments made by ship owners, many ask why more military interventions don’t take place.

NATO warships are in the area but US Navy officials have told the BBC they can "not be everywhere". That said, an Indian Navy frigate attacked and sank a Somali pirate 'mother ship' on Wednesday morning after coming under fire.

Most hijackings, however, do end peacefully although it can be a lengthy process. On Wednesday the Hong Kong ship MV Great Creation and its crew of 25 Chinese and one Sri Lankan were released after two months of captivity.