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.”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.Friends paid tribute to Jean and Scott Adam who were cruising around the world distributing bibles.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 – BBC