Four Americans hijacked by Somali pirates off the coast of Oman have been killed by their captors, US defence officials say.

The US military said its forces trailing the vessel had responded to gunfire heard aboard but found all the captives shot when they arrived.

The yacht S/V Quest, hijacked on Friday, was owned and sailed by Scott and Jean Adam of California.

Also killed were two US passengers, Phyllis Macay and Bob Riggle.

US Central Command said that negotiations were under way between the US Navy and the pirates, when the US forces heard gunfire coming from the Quest about 0600GMT.

US Navy Seal special forces sailors boarded the ship without firing a shot, then killed two pirates while they were taking control of the ship.

They discovered the four Americans shot. At least one - Ms Macay - was alive when the Seals boarded. The US Navy Seals attempted unsuccessfully to save the injured hostages, the military said.

"As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors," Gen James Mattis of US Central Command Commander said in a statement.

As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors”

Gen James Mattis
US Central Command
"We express our deepest condolences for the innocent lives callously lost aboard the Quest," the statement added.

But the BBC's Will Ross in Nairobi says the pirates' telling of the encounter differs from the US Navy's. The pirates report the US warship attacked first, killing two pirates, and the hostages were killed in retaliation.

The incident will provoke further debate on whether the use of force is wise when dealing with Somali pirates who have already taken hostages, our correspondent says.

According to the US military, four Navy warships - including an aircraft carrier - began tracking the hijacked vessel on Friday and were following it toward the Somali coast, hoping to prevent the pirates from disembarking with the hostages.

The White House said President Barack Obama on Saturday had authorised the use of force in the case of "an imminent threat" to the hostages. He was notified of the hostages' deaths soon after they were killed, spokesman Jay Carney said.

On Tuesday, Navy officials told reporters that two pirates had boarded a naval vessel for negotiations when the pirate crew aboard the Quest fired a rocket-propelled grenade at their ship.

The grenade missed, and the US Navy sailors then heard gunfire aboard the Quest and dispatched the Navy Seal boarding party, which discovered the four Americans.

The US Navy captured 13 pirates, killed two - one with shots and another with a knife - and found the remains of two other pirates already dead about the vessel, the US military said.

It was unclear how they died.

From the LA times - 

U.S. officials on Thursday defended their strategy in talks with pirates, who ultimately fatally shot four American hostages off the coast of Somalia.

U.S. officials said that negotiators told Somali pirates that that they would not be allowed to go ashore with their captives, one of several moves that increased pressure on the pirates before the hostages were killed Tuesday.

That warning was communicated early in the four-day standoff as Navy ships shadowed a yacht carrying 19 Somalis and their prisoners. The U.S. intended to prevent the pirates from taking the hostages onto Somali soil. Two couples, Jean and Scott Adam of Marina del Rey and Phyllis Macay and Robert Riggle of Seattle, were found mortally wounded when the yacht was boarded.

"The thought was, if these guys succeed in getting the hostages to shore, we have almost no leverage anymore," a U.S. Defense official said.

Some experts wondered whether the U.S. negotiators went too far in pressuring the pirates.