Four American sailors hijacked by Somali pirates off the coast of Oman have been killed, US defence officials have said.

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

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

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

In a statement, US Central Command said that negotiations were underway between the US Navy and the pirates, when the US forces heard gunfire coming from the Quest about 6am GMT.

They boarded the ship, killing two pirates in the process, and discovered the four Americans shot. The US Navy sailors attempted to provide first aid but the hostages died, the military said.

“We express our deepest condolences for the innocent lives callously lost aboard the Quest,” Gen James Mattis of US Central Command Commander said in a statement.

The US Navy captured 13 pirates, and found the remains of two other pirates already dead about the vessel, the US military said.