French commandos have stormed a yacht held by Somali pirates in an operation that left one French hostage and two gunmen dead, hours after an American skipper held in a separate ransom battle narrowly failed in a dramatic escape bid, officials said.

As a US force built up off Somalia, French forces staged their rescue six days on Friday after the yacht, the Tanit, was seized in the pirate infested Gulf of Aden, French officials said.

One male hostage and two pirates were killed in the assault on the yacht and three other adults and a three-year-old child were rescued, the officials said.

"Today, with the threats becoming more and more specific, the pirates refusing the offers made to them and the Tanit heading towards the coast, a operation to free the hostages was decided upon," a French presidential spokesperson said.

"During the operation, a hostage was unfortunately killed. The four others — including the child — are safe and sound.

"Two pirates were killed, the three others were captured," said the spokesperson.

Two couples and a child were on the yacht that was taken by armed pirates last Saturday.

French negotiators had offered a ransom, Defence Minister Herve Morin said, without giving details of the deal which would also have allowed the pirates to return to port.

France had offered to send a military officer in exchange for one woman and her child on the yacht. "All these things were permanently and constantly refused," Morin said.

Morin named the dead man as Florent Lemacon, the owner of the yacht and father of the child. He was hit during an exchange of fire between the pirates and French forces, said Morin. An investigation has been launched into the death.

French troops immobilised the yacht on Thursday by firing into the sails, said Morin.

It was the latest in a mounting number of attacks by pirates now terrorising busy international shipping lanes from the Gulf into the Indian Ocean.

During 2008, about 150 ships were attacked, according to pirate monitoring watchdogs, but there had been a lull in early 2009.

Since 4 April, Somali pirates have hijacked a US container ship, the French yacht, a British-owned cargo ship, a German container carrier, a Taiwanese fishing vessel and a Yemeni tugboat.

Some of the most spectacular successes came late last year when they seized a Ukrainian cargo loaded with combat tanks and other weaponry, as well as a Saudi super-tanker carrying $100-million in crude oil.