Pirates shot dead four US hostages on a private yacht last night, the deadliest incident involving Americans kidnapped for ransom in the increasingly dangerous waters off Somalia.
The US military said the pirates shot the hostages before American special forces boarded the vessel.
US troops killed two pirates as they took control of the boat, and took 15 pirates into custody.
Another two pirates were found dead when the US special forces arrived but they were not killed by US forces, the military said.
US secretary of state Hillary Clinton said the US government was "deeply saddened and very upset by the murder of four American citizens" - a "deplorable act" that underscored the need for more international cooperation against the pirates.
"We've got to have a more effective approach to maintaining security on the seas, in the ocean lanes, that are so essential to commerce and travel," she told reporters.
Pirate gangs preying on shipping lanes through the Gulf of Aden and the Indian Ocean typically target large merchant ships, with oil tankers the prize catch, but the snatching of foreigners can also yield high ransoms. There were around 750 pirate hostages at the end of January.
The Americans killed were Jean and Scott Adam, from California, as well as Phyllis Macay and Bob Riggle from Seattle.
The US military said negotiations with the pirates had been under way when yesterday morning, without warning, a pirate fired a rocket-propelled grenade at the guided-missile destroyer USS Sterett.
Then gunfire broke out inside the pirated vessel. "The intent always had been that this would be a negotiated process and not ever go into a point where we actually had gunfire," said Vice Admiral Mark Fox, the head of US naval forces in the turbulent region.
President Barack Obama had authorised the use of force in the case of an imminent threat to the hostages, White House press secretary Jay Carney told reporters.
In Seattle, people who knew Mr Riggle, who was in his late 60s, said he was a retired veterinarian who had worked under contract for the Seattle Animal Shelter. "He was a very nice, compassionate individual. He had a way with animals. He treated people with dignity and respect," said Dan Jordan, the director of the shelter.
Mr Jordan said Mr Riggle seemed well-suited to his companion, Ms Macay. "He was a man of few words, and she was a woman of many words," he said.
Reuters