WASHINGTON - Four Americans on a yacht hijacked by Somali pirates last week were all killed, the US military said on Tuesday.

"At approximately 1 am EST today, while negotiations were ongoing to secure the release of four American hostages, US forces responded to gunfire aboard the pirated vessel (S/V) Quest," said the US Central Command in a statement.

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The Central Command said after reaching and boarding the vessel, "the forces discovered all four hostages had been shot by their captors."

During engagement with the pirates, two of them were killed and 13 others were captured and detained along with two pirates already with US custody, said the US military, adding that remains of two other pirates already dead have also been found onboard.

"In total, it is believed 19 pirates were involved in the hijacking of the S/V Quest," said the US military.

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

The vessel was being shadowed by the US military after it was pirated on Friday.

The four Americans had been traveling with yachts participating in the Blue Water Rally after departing from Phuket, Thailand, said the organizers of the rally.

The S/V Quest broke off on February 15 after leaving Mumbai, India, to take a different route, said the organizers.