The UK is "sticking to its course" in Afghanistan amid warnings British troops could be killed in reprisal attacks for the killing of 16 civilians by a US soldier, Downing Street said.
Prime Minister David Cameron's official spokesman said the massacre appeared to be "an abhorrent act by a single individual" that would not push the international coalition off course.
The tragic incident has sparked Taliban threats of revenge against allied personnel and pushed Afghanistan firmly to the top of the agenda for Mr Cameron's talks with US President Barack Obama during this week's visit to Washington.
The Prime Minister will fly to Washington tomorrow for a two-day visit with the Obamas. The trip's organisers had tried to construct a relaxed couple of days, including a trip to a basketball match. The events of Sunday morning are likely to profoundly affect the tone of the summit.
Speaking at a press conference on Monday afternoon, Cameron said the attack was "absolutely appalling".
"We must do everything we can to make sure it doesn't in any way derail the very good work that American and British and other Isaf forces are doing in Afghanistan," he said.
"It is worth remembering why we are in Afghanistan. We are 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 is what we all dearly want.
The UK does not believe that the furious Afghan response to the killings will prompt America to accelerate Nato's agreed timetable for withdrawal, which envisages the handover of lead responsibility for security throughout Afghanistan to local forces during 2013 and the final drawdown of allied troops by the end of 2014.
Despite calls from some quarters in the US for troops to be brought home more quickly, Britain does not expect any further announcement on withdrawals from the White House during the course of this year, beyond the plan already set out by Mr Obama to pull out the 33,000-strong "surge" force in September this year.
Mr Obama has offered his condolences and promised a full investigation for what his Afghan counterpart Hamid Karzai has called an unforgivable incident.
Asked today if he feared that British lives had been placed more at risk, Mr Cameron's spokesman said the situation on the ground was for military commanders.
He told reporters: "This was clearly a tragic event. Our hearts and sympathies go out to the families who have died.
"As far as we are aware it was an abhorrent act by a single individual. There is a Nato/Isaf investigation under way.
"But we are sticking to our course and we have a clear plan which is a measured transition which will allow the Afghans to take over responsibility for security from Isaf forces."
Pressed as to the potential for reprisals against UK troops, he said: "We have a defined course and we are sticking to it. On the particular issue of UK forces, clearly that is an issue for commanders on the ground."
The former commander of British forces in Helmand, Colonel Richard Kemp, said vital trust allied forces have built up with Afghan civilians over the course of the war would be damaged.
"Not only are we likely to see protests and possibly American, possibly British soldiers killed over what happened on Saturday night, but also a very severe weakening of the relationship between many of the people in Afghanistan who were supporting us," he told ITV's Daybreak.
Col Kemp said building trust with local people was one of the most important elements of British troops' work in Afghanistan as they tried to tackle Taliban networks.
Relations between international security forces and the Afghan government would also be weakened as a result of the murders, which took place in two villages close to a US army base in Panjwai, southern Kandahar, on Saturday, he said.
A member of the US forces is believed to have turned himself in to the military authorities.
Mr Kemp added: "I think every soldier in Afghanistan, British, American and other allies, will be sickened by a person wearing their own uniform literally going door to door and killing people as they sleep in their houses.
"These are the very people that this soldier and his comrades are supposed to be in Afghanistan to protect not kill."
Latest photos from the ground in Kandahar. WARNING: Gallery contains some graphic images.