Pirates operating off the coast of Somalia have begun to use aggressive new tactics including torture against captives, according to the commander of the European Union naval force.

Captured sailors face being thrown overboard or used as human shields, locked in freezers or having plastic ties put on their genitals in the battle between international forces and Somali pirates.

"There have been regular manifestations of systematic torture," Royal Marines Commandant General Buster Howes said.

"If warships approached a pirated ship too closely, the pirates would drag hostages on deck and beat them until the warship went away."

The restraint of earlier years has been replaced with "a willingness to use violence much more quickly and much more violence".

Until now the consequences of the surge in piracy off the Horn of Africa have been largely commercial with most ransomed captives speaking of reasonable treatment.

The claims from the EU naval force suggest that Somali fishermen responding to illegal fishing in their waters made up the bulk of the initial pirate forces, but they have been succeeded by more brutal gangs.

The developments come after a hardening in tactics used against pirates. South Korean forces last month killed eight Somalis and captured five while storming a hijacked cargo ship, releasing all the hostages. The United States navy and French forces have previously killed Somalis in similar operations.

Over the past three years a multinational armada has been assembled in the waters from the Gulf of Aden to the Indian Ocean aimed at protecting commercial shipping routes. Despite this, Somali pirate gangs, using often ramshackle equipment and basic skiffs with outboard engines, have been able to hijack vastly larger craft including oil-laden supertankers.

There are 47 vessels and more than 800 seamen being held hostage off Somalia, according to independent maritime monitors Ecoterra. Eleven captured vessels are said to be deployed as "mother ships" for further raids, the EU naval force claims.

The crisis has prompted merchant ships to use high pressure water hoses and audio deterrents to fend off buccaneers. The tuna fleet based in the Seychelles now sails with marines aboard for protection and other fishing fleets have deployed armed agents including Somalis.

Last week a siege of crew members on the German-flagged MV Beluga Nomination ended with attackers executing one sailor.