Britain's Prime Minister David Cameron paid tribute to the United States and Afghan troops killed when Taliban insurgents shot down their helicopter in Afghanistan on Saturday.
"I was deeply saddened to hear of the many U.S. military personnel who lost their lives today in Afghanistan. My thoughts - and the thoughts of the whole country - are with their families and friends," Cameron said in a statement.
Marking the deadliest incident in the decade-long war, 31 U.S. special operation troops - 22 of whom were Navy SEALs from "Team Six" - the unit that carried out the raid that killed Osama bin Laden - were killed when the Taliban in eastern Afghanistan shot down their helicopter, officials said.
Seven Afghan commandos were also killed. According to sources, 22 SEALs from Team Six were killed. It was not immediately clear whether any took part in the raid that killed the 9/11 mastermind.
"They have made the ultimate sacrifice in helping to protect our security, and to build a moer stable and peaceful Afghanistan. We remember, too, the Afghan troops who died alongside them today," Cameron said. "Britain, and our own armed forces serving in Afghanistan, will continue to work steadfastly alongside their U.S. and Afghan colleagues as they help prepare Afghanistan to secure its own territory."
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Britain has about 9,500 troops in Afghanistan, making it the second-largest contributor to ISAF after the United States. But Cameron wants all British troops out of a combat role in Afghanistan by 2015.
Responsibility for the helicopter incident was claimed by the Taliban; the crash occurred in Wardak province, just west of the capital, Kabul.
The U.S. official, who spoke on condition of anonymity because he was not authorized to discuss the matter, said the aircraft was most likely brought down by rocket-propelled grenade.
The attack occurred after a night raid, a method that has been praised by American commanders as one of the most effective in the recent military offensive, despite the raids being heavily criticized by Afghan officials and civilians.
NATO confirmed the overnight crash took place and that there "was enemy activity in the area." NATO said it was still investigating the cause, however, and conducting a recovery operation at the site.
"We are in the process of accessing the facts," said U.S. Air Force Capt. Justin Brockhoff, a NATO spokesman.
U.S. President Barack Obama offered his condolences and prayers to the families of the Americans and Afghans who died in the attack, saying their deaths were a "reminder of the extraordinary sacrifice made by the men and women of our military and their families, including all who have served in Afghanistan," Obama said.
"We will draw inspiration from their lives, and continue the work of securing our country and standing up for the values that they embodied," he said.
"We also mourn the Afghans who died alongside our troops in pursuit of a more peaceful and hopeful future for their country."