The hijacking of ships near the coast of Somalia has cost the shipping industry millions of dollars [AFP]

Five Somali men, convicted of attacking a US Navy ship, have been sentenced to life in prison by a Virginia court.

Tuesday’s sentencing is the harshest yet for accused pirates as the US tries to halt piracy off Africa's coast.

The federal prosecution relied upon rarely-used 19th century maritime laws, and was the first piracy case to go to trial since the Civil War, when a New York jury deadlocked on charges against 13 Southern privateers.

The five Somali men were convicted on federal piracy charges on November 24 last year.

Prosecutors argued during trial that the five had confessed to attacking the USS Nicholas on April 1 after mistaking it for a merchant ship.

Defence lawyers had argued the men were innocent fishermen who had been abducted by pirates and forced to fire their weapons at the ship.

However, John S Davis, an assistant US attorney, had argued that three of the men were in a skiff that opened fire on the Nicholas with assault rifles, then fled when sailors returned fire with machine guns.

Davis said all the men later confessed to the attack in remarks to an interpreter on board the ship. He said they expected to make anywhere from $10,000 to $40,000 from the ransom, a comparatively small sum.

Defiant pirates

The attorneys argued that the men - Gabul Abdullah Ali, Abdi Wali Dire, Abdi Mohammed Gurewardher, Abdi Mohammed Umar and Mohammed Modin Hasan - had actually hoped to be rescued.

Last month, a Somali pirate who kidnapped and brutalised the captain of a US-flagged merchant ship off the coast of Africa in 2009 was sentenced to more than 33 yearsin prison.

The hijacking of ships near the coast of Somalia has cost the shipping industry millions of dollars.

Pirates have continued to attack foreign ships in the Gulf of Aden and the Indian Ocean, defying an armada of warships trying to protect the key maritime route.

The fight against piracy has been hampered by legal ambiguities over the appropriate venue to prosecute captured suspects.