ABOARD THE USS MAHAN - American warships off the lawless Somali coast are using unmanned drones to hunt pirates threatening one of the world's most important shipping lanes.

Sailors aboard the USS Mahan told The Associated Press they have been using the spy flights daily to spot potential pirate mother ships.

For years, the U.S. has used drones to track potential terrorists among Somalia's warlords, but the navy said more and more of the planes are now being used to fight piracy.

The drones can fly more than 900 metres above sea level and relay pictures detailed enough to recognize the flags flown on fishing boats that Somalis use to avoid detection.

The drones take still photos and videos that are instantly relayed to the American ships. The ships can then send this material to other countries in the international anti-piracy coalition that may have ships near the suspicious vessel. Countries as diverse as India, France, China and Russia have sent ships to help patrol the Gulf of Aden.

On Thursday, pictures taken by the drones, some of which also are equipped with night vision, helped apprehend nine pirates after a night flight relayed pictures of a skiff with a ladder onboard. A skiff had fired a rocket-propelled grenade at a merchant vessel in the area earlier.

The American warship dispatched helicopters to provide surveillance and air cover, and it deployed a boat with a search and seizure team.

Automatic weapons and rocket-propelled grenades were found and the nine men onboard were detained, although they had thrown the ladder into the sea. Still, the pictures of the ladder taken by the drone can be used as evidence, as the coalition steps up efforts to pursue the pirates through the courts as well as the waves.

Pirate mother ships often are used to tow smaller skiffs out to sea and resupply them.

Previous anti-piracy efforts have been hindered by confusion over which country has the jurisdiction to prosecute suspected pirates, but the United States and Britain both signed an agreement with Kenya to try suspects in that country, which borders Somalia.

"We have a unique capability in which we have an (unmanned air vehicle) that helps us detect the pirates and makes it hard for them to hide," USS Mahan Capt. Stephen Murphy said, pointing to the images the drone relayed to the bridge of the destroyer.

"The UAV ... can stay airborne all day and cover thousands of miles (kilometres) of the ocean and be able to spot pirates," he said.

Somali pirates have been preying on passing shipping for years, but September's capture of a Ukrainian ship loaded with arms helped focus international attention on the problem. The arms ship was released earlier this month and docked in a Kenyan port Thursday.

Pirates attacked more than 100 ships last year with a success rate of nearly 50 per cent.

The number of attacks has remained steady following an influx of warships into the Gulf of Aden late last year, but their success rate has fallen to below 30 per cent.

There also has been a recent spate of unseasonably bad weather.

But analysts say the problem will not be solved until a stable government is established in war-ravaged Somalia. The country has not had one since 1991, and the multimillion dollar ransoms are a strong lure in a country where nearly half the population is dependent on aid.

The embattled United Nations-backed government is fighting a strengthening Islamic insurgency that the U.S. State Department says has links to al-Qaida.