KABUL - A US soldier accused of shooting dead 16 Afghan civilians has been flown out of Afghanistan, officials said, as Washington attempted to calm seething anger over a massacre that raised serious questions about the West's war strategy.Underscoring the instability in Afghanistan, an Afghan man in a stolen pickup truck sped onto the tarmac as a plane carrying US Defense Secretary Leon Panetta was about to land on Wednesday, an extraordinary security breach in a southern province next to where Sunday's massacre took place.The New York Times, citing an unidentified senior US official, said the soldier had been flown to Kuwait. CNN also reported the sergeant had been taken there.The commander of U.S. and Afghan forces in Afghanistan, General John Allen, made the decision based on a legal recommendation, a US official said.Afghan President Hamid Karzai's office was understood to have accepted that the soldier be tried in a US court, provided the process was transparent and open to media.Panetta, the most senior US official to visit Afghanistan since the massacre, told US troops it must not deter them from their mission to secure the country ahead of the 2014 NATO deadline for the withdrawal of foreign combat troops."We'll be challenged by our enemy. We'll be challenged by ourselves. We'll be challenged by the hell of war itself. But none of that, none of that, must ever deter us from the mission that we must achieve," Panetta told soldiers at Camp Leatherneck, the main US Marine base in Helmand.US and other foreign soldiers listening to Panetta had been asked to leave their weapons outside, a highly unusual move that was downplayed as a gesture to Afghan troops who were unarmed during the address.Tensions have risen sharply across Afghanistan since the attack and the inadvertent burning of copies of the Koran at the main NATO base last month, adding urgency to Panetta's visit. Panetta was to hold talks with Karzai and other Afghan leaders.The Afghan Taliban threatened to retaliate for Sunday's shooting by beheading US personnel, while insurgents also attacked investigating Afghan officials on Tuesday.Obama, Cameron 'remain committed' to completing Afghan missionIn Washington, President Barack Obama said after meeting British Prime Minister David Cameron he did not anticipate any sudden change in plans for the pace of withdrawing troops.Obama described the Kandahar massacre as tragic but emphasized at a briefing with Cameron that both nations remained committed to completing the Afghan mission "responsibly"."In terms of pace, I don't anticipate at this stage that we're going to be making any sudden additional changes to the plan that we currently have," Obama said.NATO leaders gathering in Obama's home city of Chicago on May 20-21 will decide the next phase of the planned transition to Afghan forces, which is already under way.Obama acknowledged that people wanted the war over, but argued they still back the reason for troops being there.In a Reuters/Ipsos poll, 40 percent of Americans said the shooting had weakened their support for the war.No one on board the military plane carrying Panetta was hurt when it landed at a British base in Helmand province. Defense officials played down the incident, saying the Pentagon chief was never in danger.The pickup truck crashed into a ditch after it sped across the runway ramp and the driver, whose motives were unclear, emerged from the vehicle in flames.He was being treated for burns, a Pentagon spokesman said, and a member of the NATO-led coalition was also hurt when the vehicle was stolen.Panetta arrived for his unannounced visit three days after the massacre in neighbouring Kandahar province. Although Panetta's trip was planned before the shooting, it comes as Afghan civilians and lawmakers alike demand answers.Foremost among those demands is that the soldier responsible be tried in Afghanistan over the shooting, one of the worst of its kind since US-backed Afghan forces toppled the Taliban in 2001 for harboring the al-Qaida masterminds of the Sept. 11 attacks on the United States.Despite those calls, the US staff sergeant who gave himself up after the villagers, including nine children and three women, were killed has been flown out of Afghanistan, the Pentagon said on Wednesday.