Three soldiers killed and two injured in US 'friendly fire' incident.
The apparent "friendly fire" incident took place as the troops patrolled an area north-west of Kajaki, Helmand province, on Thursday night.
After coming under attack the unit, from 1st Battalion The Royal Anglian Regiment, called in close air support from two US F15s.
One of the aircraft launched a bomb, which apparently exploded hitting five British troops, the Ministry of Defence said yesterday.
Three soldiers were declared dead at the scene and two others were evacuated to Camp Bastion with serious injuries.
The commander of the Helmand task force, Brigadier John Lorimer, said the deaths were a "terrible tragedy".
"It is all the more devastating for everyone because we have been working with the US Air Force a great many times during the time that we have been out here," he said.
"The bombs dropped by American aircraft have saved the lives of hundreds of British troops."
Lieutenant Colonel Charlie Mayo, spokesman for British troops in Helmand province, said an investigation into the deaths had been launched.
"There are a handful of different reasons why this tragic incident has happened and we are not in a position at the moment and I don't think we will be for some time - to find out exactly what has - happened," he said.
This is believed to be the second friendly fire incident involving British troops in Afghanistan.
The MoD is still investigating reports that 21-year-old Royal Marine Jonathan Wigley was the victim of allied fire when he was killed in Helmand in December.
A string of friendly fire deaths in Iraq prompted defence officials to propose setting up special combat simulators to train US and UK troops involved in joint operations.
Eight UK service personnel have become accidental victims of allied forces since the start of the conflict in 2003, according to the MoD.
Most famous is the case of Lance Corporal Matty Hull, who was killed after a US aircraft fired on two armoured vehicles in March 2003.
L/Cpl Hull, 25, of the Household Cavalry Regiment, died from multiple injuries inside his blazing Scimitar armoured car despite colleagues' efforts to save him.
He was travelling in a column of light armoured vehicles near Basra when it was attacked by a US A-10 "tankbuster" aircraft.
Other UK soldiers who survived the attack criticised the US pilot for apparently failing to recognise that their tanks were a British make, with special coalition identification aids and even a large Union Flag on another machine in the five-vehicle convoy.
The US initially refused to release a classified cockpit recording of the incident, only relenting when a British newspaper obtained a copy and published it on the internet.
The US has also refused to send personnel involved in friendly fire deaths to UK inquests.