Pirates shot dead four American hostages on a yacht they had seized in the Arabian Sea, and a firefight left two pirates dead and 13 captured, the U.S. military said on Tuesday.

The sequence of events was not immediately clear, but the U.S. military's Central Command said the dead hostages were only discovered after U.S. forces responded to gunfire and boarded the pirated yacht, known as the Quest.

As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors, the U.S. military's Central Command said in a statement.

Despite immediate steps to provide life-saving care, all four hostages ultimately died of their wounds.

The military, which said the incident took place at about 1 a.m. EST/0600 GMT, had been monitoring the Quest since discovering it had been taken over by pirates for about three days. It said negotiations to secure the release of the Americans had been under way when the gunfire broke out.

The U.S. Embassy in Nairobi on Saturday said a yacht with four Americans on board was hijacked in the Arabian Sea, and was heading toward Somalia.

Two Somali pirates spoke with Reuters by telephone on Tuesday.

Our colleagues called us this morning, that they were being attacked by a U.S. warship, a pirate who identified himself as Mohamud told Reuters.

The U.S. warship shot in the head two of my comrades who were on the deck of the yacht by the time they alerted us, Mohamud said. This is the time we ordered the other comrades inside yacht to react -- kill the four Americans because there was no other alternative -- then our line got cut.

The killing of those four Americans and our comrades is a fair game that has started. Everybody will react if his life is in danger. We should not agree to be killed and let the hostages be freed, a pirate called Hussein told Reuters from Hobyo, another Somali coastal pirate haven.

Neither could say how many of their colleagues were killed.

Pirate gangs preying on shipping lanes through the Gulf of Aden and the Indian Ocean typically target large merchant ships, with oil tankers and the prize catch, but the snatching of foreigners can also yield high ransoms.

Somali pirates Somalia Somalia U.S.