David Cameron: Afghan killings 'absolutely dreadful'
David Cameron has described the US soldier's killing of 16 Afghan civilians as "absolutely dreadful".
He said that "one's heart just goes out to those families in Kandahar who've suffered these appalling losses".
But the prime minister insisted the UK, the US and other allies must "stick to the plan" set out for Afghanistan and not allow it to "derail" their work.
The government said the UK military was "always on the lookout" for signs of psychological stress among personnel.
The US soldier, believed to be a staff sergeant, left his base in Kandahar in the early hours of Sunday.
Locals say he broke into three homes. Eleven people were shot dead at one house in the village of Najeeban, and some of their bodies had been set alight.
At least three of the child victims are reported to have been killed by a single shot to the head.
The soldier returned to his base after the shootings and apparently turned himself in to military authorities.
During a joint press conference with Irish Prime Minister Enda Kenny, Mr Cameron said: "It really is an absolutely appalling thing that has taken place, and of course it will have its impact.
"But we must do everything we can to make sure that it doesn't in anyway derail the very good work that American and British and other ISAF (International Security Assistance Force) forces are doing in Afghanistan.
"It is worth remembering why we are in Afghanistan. We're there to train up the Afghan army and police so that that country is able to look after its own security and make sure that country isn't a haven for terrorists without having foreign troops on its soil. That's what we all dearly want."
There has been widespread anger in Afghanistan after the shootings.
President Hamid Karzai called them "unforgivable", while the country's MPs have warned Afghans "have run out of patience" with foreign troops.
Mr Cameron, speaking ahead of a visit to the US, was asked whether he might discuss a change in strategy for Afghanistan when he met the President Barack Obama.
He said: "We have a plan that is about transitioning Afghanistan over to Afghan control. That plan applies in Helmand as much as anywhere else, and I think the most important thing is we stick to that plan, we deliver that plan.
"And then we can bring our troops home, having done a good job in giving Afghanistan at least a chance of stability and prosperity and growth for the future.
"That is what we must do. It is very difficult. My first concern of course is with British troops, and making sure they are properly protected, and making sure that they have all the he equipment and everything they need.
"We should pay tribute to them. They do an amazing job, (at) what is obviously a difficult time, but we must stick to the plan and deliver the plan as we set it out."
In the House of Commons, Labour MP David Winnick asked an urgent question on the shooting incident, saying there was a "growing sense in Britain that this is an unwinnable war".
Former Lib Dem leader Sir Menzies Campbell said that, as potential service personnel, had to undergo rigorous physical checks, testing should be "equally searching" when it came to their mental state.
Armed forces minister Nick Harvey replied: "We are always on the lookout for signs of people suffering psychological stress. That would occur at every point.
"Considerable progress has been made in recent years in removing the sense of stigma."
He added: "There is no question of us cutting and running early (from Afghanistan), either because of these events or any others."