Taliban insurgents have threatened to carry out revenge attacks after a US soldier went on a shooting rampage in Afghanistan, killing at least 16 civilians.
Nine children and three women were among those killed, shot as they slept in their beds. Four men were also killed and five others were wounded.
Afghanistan's President Hamid Karzai described the attacks as "impossible to forgive", saying in a statement on Sunday:
"When Afghan people are killed deliberately by US forces this action is murder and terror and an unforgivable action."
There are fears of reprisals on both British and American soldiers after a statement on the Taliban website described the attack as committed by "sick minded American savages" and vowed to "take revenge from the invaders and the savage murderers for every single martyr" news agency AFP reports.
Colonel Richard Kemp said the massacre will also cause an erosion of the vital trust allied forces have built up with Afghan civilians over the course of the war, and that the attacks could result in revenge attacks on British soldiers.
He told ITV's Daybreak: "One of the most important things that our forces do day-to-day in Afghanistan is to build up trust with the local people and get them to turn against the Taliban and provide extremely important intelligence that enables us to take the Taliban networks apart.
"That trust is going to be eroded by this kind of incident, so 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."
The former commander said 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.
President Barack Obama has phoned his Afghan counterpart to offer his condolences and expressing his "shock and sadness" at the massacre while military officials vowed to bring those responsible to justice.
An investigation has been launched into the incident, which was described by the UK ambassador to Afghanistan as a "completely out-of-the-ordinary event".
The service member, an Army staff sergeant, is being detained in Kandahar, in the south of the country, with indications suggesting he handed himself in following the massacre.
ABC reports that the sergeant was a '38-year-old father of two' and there has been some speculation that the soldier was drunk when the attacks took place.
Sir William Patey, who retires from the job next month, told BBC Radio 4's The World This Weekend the deaths in southern Afghanistan had nothing to do with Nato operations.
He said: "It's obviously clear from what we know so far, and we obviously have to await the outcome of an investigation, that this looks like the apparent act of a single individual and is no part of any Nato/Isaf operation. So we proceed in that context.
"This is not something that has characterised Nato/Isaf's presence over the last 10 years in Afghanistan. It's a unique event, out of the blue."
The shootings took place in the villages of Balandi and Alkozai in Panjwai, southern Kandahar, about 500 yards away from a US base.
In a statement released by the White House, President Obama said: "This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan."
Nato officials also apologised for the killings, which have sparked fears of reprisal attacks against allied troops serving in the country.
Lieutenant General Adrian Bradshaw, deputy commander of Nato forces in Afghanistan, said: "I wish to convey my profound regrets and dismay at the actions apparently taken by one coalition member in Kandahar province.
"One of our soldiers is reported to have killed and injured a number of civilians in villages adjacent to his base. I cannot explain the motivation behind such callous acts but they were in no way part of authorised Isaf military activity."
The shooting is certain to raise tensions in the region further, after mass protests followed the burning of the Koran by two American soldiers at the end of last month. Some analysts viewed the book-burning as a "tipping point" for the country, likely to substantially increase support for the Taliban.
The deteriorating security situation led to the British embassy removing all non-essential staff from its offices in the Afghan capital, Kabul.
Sky News quotes the US embassy in Kabul as saying: "We deplore any attack by a member of the US armed forces against innocent civilians, and denounce all violence against civilians.
"We assure the people of Afghanistan that the individual or individuals responsible for this act will be identified and brought to justice."
WARNING: Gallery contains some graphic images
An Afghan man sits in the back of a bus with the body of a person who was allegedly killed by a U.S. service member in Kandahar province south of Kabul, Afghanistan.
An Afghan soldier is seen at a guard tower at a military base as civilians gather outside in Panjwai, Kandahar province south of Kabul, Afghanistan
President Barack Obama talks on the phone with Afghanistan President Hamid Karzai from his vehicle outside the Jane E. Lawton Community Center in Chevy Chase
U.S. Army and Afghan soldiers are seen in a guard tower at their base in Panjwai, Kandahar province south of Kabul, Afghanistan.
A villager points to a spot where a family was allegedly shot in their residence by a rogue US soldier in Alkozai village of Panjwayi district, Kandahar province.
US soldiers keep watch at the entrance of a military base near Alkozai village following the shooting of Afghan civilians allegedly committed by a rogue US soldier in Panjwayi district, Kandahar province on March 11, 2012.