French sailors have arrested 11 gunmen after a warship intercepted a ship off the coast of Kenya as part of an anti-piracy operation, France's defence ministry has said.

The French vessel, which had been patrolling waters off East Africa, chased the pirates 926km after first observing them on Tuesday night.

A defence ministry spokesman, who said the pirates had been sailing a 10-metre "mother ship," said the captives were being detained on board the Nivose, the French warship.

"The frigate spotted the pirates on the evening of April 14 when its helicopter intervened to thwart an attack on the merchant ship Safmarine Asia."

"She followed their boats overnight and intervened at dawn."

The Nivose is part of a European Union anti-piracy force involving warships from France, Germany, Spain and Italy, which aims to aid international efforts to curb a spate of Somali pirate attacks.

In a separate incident on Wednesday, Somali pirates released a Greek cargo ship and its 24-man crew, which had been seized on March 19 in the Gulf of Aden.

"The ship has been released and the crew are in good health," a ministry spokeswoman told the AFP news agency.

The St Vincent-flagged Titan, which was carrying iron, had been sailing from the Black Sea to South Korea when it was taken over by pirates.

Rescue operation

The release of the ship comes a day after three Somali pirates detained by the French military last week were taken into custody in France.

Herve Pavy, a French prosecutor, said: "The three Somali pirates apprehended [on April 10] were brought onto national territory and taken into custody on April 14".

The trio were arrested during a rescue operation by French special forces on Friday to free a group of hostages on board the Tanit, a French yacht.

The Tanit's owner died in the gunfight which took place during his family's rescue [AFP]

Four hostages, including the widow and three-year-old son of Florent Lemacon, the yacht's owner, were freed.

Lemacon died in the gunfight which took place during the rescue.

Meanwhile, the supreme court in Somalia's northern breakaway state of Puntland has handed down three-year prison terms to 37 pirates detained by the French and US navies.

Mohamed Abdi Aware, the judge, said: "After listening to the charges against the defendants, who were accused of being armed gangs attacking ships, the court recognised them as criminals and sentenced them to three years each."
 
The same court, in the port city of Bosasso, had jailed another 15 pirates to three years in prison last week.

Of those sentenced on Wednesday, 19 had been handed over by the French navy and 18 by the US navy, said a court official.

The pirates denied the piracy charges in court, claiming they were "fishermen illegally arrested" by foreign navies.

Clinton statement

Meanwhile, Hillary Clinton, the US secretary of state, has announced new diplomatic efforts to freeze the pirates' assets and said the US administration will work with shippers and insurers to improve their defences against pirates.

"These pirates are criminals, they are armed gangs on the sea. And those plotting attacks must be stopped," she said at the state department in Washington on Wednesday.

Clinton did not call for military force, although she mentioned "going after" pirate bases in Somalia, as authorised by the UN several months ago.