The apparent killing of 16 Afghan civilians by a US soldier came "out of the blue" and had nothing to do with Nato operations, the UK ambassador in the country said today.

Sir William Patey, who retires from the job next month, told BBC Radio 4's The World This Weekend that the shooting of villagers in southern Afghanistan was a "completely out-of-the-ordinary event".

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 soldier apparently walked out of a base earlier today and started shooting Afghan civilians.

NATO officials have apologised for the attack.

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 took place in two villages in Panjwai, southern Kandahar: Balandi and Alkozai. The villages are about 500 yards away from a US base.

In the interview, Sir William also insisted British forces would continue to see through the Government's military commitment until 2014 when troops will start returning home.

He added: "On the general point, we have a plan and a strategy to gradually withdraw from Afghanistan our combat troops in line with them being replaced by Afghans.

"It is right. The Afghans are a proud people and there comes a point when the Afghans want to see their own forces on the streets performing security functions. That's what our plan aims to do and with 18 months, two years, that plan will have been implemented.

"Many Afghans say to me that we might go out too quickly. They are concerned that if we go out too quickly, there might be a vacuum."