LONDON - International naval patrols off Somalia and armed private guards on board ships have driven pirate attacks to a five-year low in 2012, but the risk to shipping off west Africa is growing.

Somali piracy in the busy shipping lanes of the Gulf of Aden and the northwestern Indian Ocean off east Africa cost the world economy some $7 billion in 2011, according to the American One Earth Future foundation.

Global pirate attacks on ships fell to 297 in 2012, compared with 439 in 2011, and was at its lowest since 2008 when 293 incidents were recorded, watchdog the International Maritime Bureau (IMB) said on Wednesday.

About 10 percent of those attacks resulted in the ship being successfully hijacked. Twenty eight vessels were taken in 2012, down from 45 in 2011 and 53 in 2010.

Of those 28, 14 were commandeered by Somali gangs, half the number taken in 2011, said the IMB, which has been monitoring global piracy since 1991.

"The continued presence of the navies is vital to ensuring that Somali piracy remains low," said IMB director Pottengal Mukundan. "This progress could easily be reversed if naval vessels were withdrawn from the area."

Last week a Somali pirate kingpin nicknamed "Big Mouth" said he had renounced a life of seizing ships after making millions in ransoms and urged Somali youth not to take to the high seas, in a sign of the effectiveness of the crackdown.

Prompted by soaring costs for shippers, including insurance and the safety of their crews, international navies stepped up pre-emptive action against the pirates, such as strikes on their bases on the Somali coast.

Shipping firms are also increasingly using guards and other measures such as better monitoring and razor wire.