WASHINGTON – A federal grand jury in the United States has indicted 14 suspected pirates over a boat hijacking last month that ended with the four American hostages being shot dead, officials said Thursday.

"Fourteen individuals -- 13 Somalis and one from Yemen -- have been indicted in relation to the pirate attack and murder of four Americans on the Quest sailing vessel," Peter Carr, a spokesman for the US attorney's office in Virginia, told AFP.

At least three of the defendants are accused of killing the Americans, the first to die in a spate of kidnappings in recent years that has focused world attention on the pirate-infested waters off Somalia.

The Americans were killed "without provocation before the hostages could be rescued by members of the military, who had been attempting to secure the release of the hostages through negotiations with the defendants," said the indictment, dated Tuesday.

The suspects face charges of piracy, kidnapping and firearms offenses, "but not murder at this time," Carr said.

The men were allegedly part of a band of pirates that commandeered the 58-foot (17.5-meter) Quest as it sailed in in the Indian Ocean near Somalia in mid-February.

The pirates and the US authorities have offered sharply different versions of events.

According to Vice Admiral Mark Fox, head of the US Naval Forces Central Command based in Bahrain, two of the pirates had been brought onboard a nearby US warship to conduct negotiations to free the hostages.

Then, with "absolutely no warning," the pirates launched a rocket-propelled grenade at the warship, the USS Sterett, though several Somalis also raised their arms in surrender on the yacht's deck, Fox said.

US Special Forces raced to the yacht on small boats. By the time they boarded, they heard gunfire and saw that all four Americans had been shot, Fox said. They died after efforts to treat them failed.

However, a senior commander from the pirate lair in Garacad, in Somalia's northern self-declared state of Puntland, told AFP a US naval assault had provoked the cross-fire.

Last month, a US judge sentenced a teenage Somali pirate to nearly 34 years in prison for his part in the 2009 hijacking of another US ship, the Maersk Alabama.

That incident had a more successful outcome for US special forces, who freed the ship's captain, Richard Phillips, in an operation that killed three pirates.

In another deadly military intervention, French forces launched a commando operation to free a yacht held by pirates in 2009 and rescued a small child and his mother but killed his father.

The multi-billion-dollar naval deployments in the region have failed to stem piracy, which is currently at an all-time high, with more than 40 vessels and 800 hostages in pirate hands.

The Danish foreign ministry said Monday that Indian Ocean pirates had captured a yacht last week with seven Danes, including three children.

Pirates holding the Danes killed eight Puntland government troops on Thursday who were heading towards their mountain hide-out, security officials said.