The US has begun an investigation into the deaths of three British soldiers killed in Afghanistan by "friendly fire" from US fighter planes.
The US State Department pledged a thorough inquiry and said it was "deeply saddened".
It is understood the soldiers, from 1st Battalion The Royal Anglian Regiment, died when US fighters dropped a bomb near their patrol in Helmand province.
The men are expected to be named by the Ministry of Defence later.
Two other soldiers were injured. The next of kin have been informed.
The US State Department's Kurt Volker said in a statement: "The United States is deeply saddened by the death of three British soldiers in Afghanistan and the wounding of two others in a friendly fire incident involving ordnance dropped by a US aircraft."
Mr Volker, the US principal deputy assistant secretary of state for European and Eurasian Affairs, added: "We will investigate this tragic incident thoroughly with our British allies.
"We are committed to making information available as quickly as possible."
The US Embassy in London said in a statement: "The United States expresses its deep condolences to the families and loved ones of the soldiers who died and we wish those who were injured a speedy recovery."
"The UK soldiers were serving under the Nato-led International Security and Assistance Force (ISAF) which is helping the Afghan people to build a peaceful, prosperous, and stable country."
The MoD said all the soldiers had been taking part in a fighting patrol to disrupt Taleban activity to the north west of Kajaki, in Helmand province.
They came under attack from Taleban insurgents and during the firefight that followed, close air support was called in from two US F15 aircraft, the MoD said.
Defence Secretary Des Browne said he was "very saddened" by the death of "three brave men who were tragically killed in what is believed to be a friendly fire incident".
He said such incidents were rare and the latest tragedy would be thoroughly investigated.
He added that US air support was very important and had often helped get British troops out of dangerous situations.
"We go to extraordinary lengths to ensure these things don't happen but at the end of the day combat environments are very complex environments. Human error is always a possibility," he said.
Lieutenant Colonel Charlie Mayo, a spokesman for British troops in Helmand province told BBC Radio 4's The World at One: "During this patrol they came into contact with some Taleban from a number of firing positions.
"As they came under fire they then called in some close air support to assist them and an aircraft came in, it dropped a bomb and tragically this bomb killed three of the soldiers and injured two more."
The three soldiers were declared dead at the scene. The injured two were evacuated by helicopter to the medical facility at Camp Bastion.
Lt Col Mayo said one of the two was very seriously wounded and the other seriously wounded.
BBC Kabul correspondent Alastair Leithead said close air support was relied on by ground troops to provide cover, attack the enemy and to get them out of trouble.
"In the heat of battle when support is needed very quickly, nine times out of 10 it really does help to get troops out of very difficult positions," he said.
He said the investigation would focus on the breakdown in communications between ground and air forces.
He added that there is no single system that coordinates the air-to-ground cover technologies used by all the nations that make up the Nato force in Afghanistan.
The only other case of British service personnel being killed by friendly fire involving American military personnel in Afghanistan is still under investigation.
Royal Anglian fatalities
An inquiry was launched in December 2006 into the death of marine Jonathan Wigley, 21, killed during an intense battle in Helmand province.
In February, Mr Browne told MPs that since 1990, 12 UK service personnel had been killed in friendly fire incidents involving American military personnel in Iraq.
The three deaths on Thursday mean the total number of UK troops killed while on operations in Afghanistan since 2001 has risen to 73. Fifty have been killed in action.
The Royal Anglians have been one of the regiments hardest hit by the fighting in Afghanistan.
Nine soldiers members of the regiment have been killed. It is one of the worst casualty rates since Operation Herrick, the campaign in Afghanistan, began in 2001.
On 11 August the regiment's Captain David Hicks was killed during an attack by the Taleban on his patrol base northeast of Sangin in Helmand province.
The regiment recruits mainly from the counties of Essex, Suffolk, Cambridgeshire and Norfolk.