Shipping lanes off the Horn of Africa will continue to be patrolled by maritime forces of NATO-members until 2016 to protect against piracy, the alliance has announced.

Operation Ocean Shield is an international effort to curb pirate attacks against commercial vessels sailing in the Indian Ocean and has been in operation since 2009.

In 2011, 24 ships were captured by pirates and 129 were attacked off the coast of Somalia. The number dropped to just 20 attacks in 2012 as a result of NATO patrolling.

Ships from Spain, Italy and Turkey are currently part of the NATO fleet which operates roughly from the Arabian Gulf to the north, the Seychelles in the south, the Gulf of Aden to the west and the Maldives in the east, covering an area greater than 2 million square miles or the size of Western Europe.

"NATO vessels verify the activity of shipping off Somalia, separating out legitimate maritime traffic from suspected pirate vessels," NATO said. "Commercial ships that are transiting the area are in many cases escorted to ensure their safe passage.

"NATO ships can actively pursue suspected pirate ships to prevent attacks. NATO boarding teams can board a suspect ship to determine if pirates are on board. As a means of last resort, NATO vessels can use force to stop pirates. "

The decision to extend the operation until 2016 was made earlier this week by NATO defense ministers.

It's estimated that maritime piracy costs the world economy about $18 billion a year.