Somali pirates killed four Americans, including a retired couple, onboard their hijacked yacht in a sudden violent turn to efforts to end a hostage drama, the US military said.
Four Somali pirates also died, two of them killed by US special forces in one of the deadliest endings to a raft of hostage-takings off the coast of Somalia that are often resolved through ransom payments.
Jean and Scott Adam, a California couple active in missionary work, had been sailing the world on the S/V Quest yacht for more than seven years and had planned to take in sites from India to Djibouti to Crete on their latest trip.
Phyllis Macay and Bob Riggle, a couple from Seattle who joined the Adams on the 58-foot (17.5-meter) yacht, were also killed by the band of 19 pirates who commandeered the yacht in waters southeast of Oman, US officials said.
Secretary of State Hillary Clinton said the United States "strongly condemns the murder" of the four and urged other nations, particularly in Africa, to contribute to efforts to fight piracy from lawless Somalia.
"This deplorable act firmly underscores the need for continued international progress toward confronting the shared security challenge posed by piracy in the waters off the Horn of Africa," Clinton said in a statement.
President Barack Obama had authorized the use of force in case of an imminent threat to the hostages and was informed at 4:42 am (0942 GMT) of the "tragic outcome," his spokesman Jay Carney said.
Four US warships, including the nuclear-powered USS Enterprise aircraft carrier, had been tracking the S/V Quest since a Danish crew discovered that it had been hijacked three days earlier, US officials said.
The US military brought two of the pirates onboard the USS Sterett on Monday to conduct negotiations to free the hostages, said Vice Admiral Mark Fox, head of the US Naval Forces Central Command based in Bahrain.
Then Tuesday morning, with "absolutely no warning," the pirates launched a rocket-propelled grenade at the USS Sterett, though several Somalis also raised their arms in surrender on the yacht's deck, Fox said.
US Special Forces raced to the yacht on small boats. By the time they boarded, they heard gunfire and saw that all four Americans had been shot, Fox said. They died after efforts to treat them failed.
The US forces soon took control of the yacht, stabbing to death one pirate and shooting dead another, Fox told reporters at the Pentagon by telephone.
Two more Somali pirates were found dead inside the vessel in unclear circumstances, possibly the result of an earlier fight among the hijackers or of the bullets that killed the hostages, according to US officials.
The Adams had been active in missionary work. On a previous yacht tour of islands across the South Pacific, the couple stopped off to distribute Bibles.
"Jean and Scott were faithful people," said Father Lloyd Torgerson, pastor of Saint Monica Catholic Church near Los Angeles where the couple was active.
"They were people that worked hard all their lives and decided in their retirement that they wanted to do something to make a difference in this world," he said.
Clayton Schmit, a friend of Scott Adam, told CNN the couple knew the trip could be dangerous but had had "this tremendous heart" for religious missions.
In San Francisco, Phyllis Macay's niece Nina Crossland said she came from a large, close family. "We love her and we are devastated," Crossland told reporters.
The couple had kept a blog on their latest trip, which began in early January at the Thai resort of Phuket. Jean Adam described Djibouti, past Somalia and the Gulf of Aden, as "a big refueling stop."
"I have NO idea what will happen in these ports, but perhaps we'll do some local touring," she wrote.
The waters near Somalia have been plagued by piracy in recent years, with hijackings a key source of income for a country that has lacked a functioning government for two decades.
The 15 surviving pirates from the clash are being held on a US ship and will eventually face judicial action "to hold them accountable," Fox said.
Just last week, a US judge sentenced a teenage Somali pirate to nearly 34 years in prison for his part in the 2009 hijacking of another US ship, the Maersk Alabama.
That incident had a more successful outcome for US special forces, who freed the ship's captain, Richard Phillips, in an operation that killed three pirates.
While few Americans have been caught up in such hijackings, a number of European yacht enthusiasts have been captured by Somali pirates.
In November, British couple Paul and Rachel Chandler were released after an ordeal that lasted more than a year.
"The intent was always that this would be a negotiated process and not ever going to the point where we actually had gunfire," Fox said.
A group called the Thailand to Turkey Convoy, or TTT, said it had feared such a deadly incident and had been trying to persuade yachts to travel together for protection.
"Hopefully, their death is not in vain and will contribute to safer waters in the Indian Ocean," said Rene Tiemessen, a leader of the effort.