Kabul, Afghanistan ? A U.S. warplane mistakenly dropped a 500-pound bomb on British troops after they called for air support in Afghanistan, killing three soldiers and seriously wounding two others in an accident that could re-ignite debate about America’s heavy use of air power.

Friendly fire involving U.S. troops has led to the deaths of three British servicemen in the current Iraq war, but the incident Thursday night was the first confirmed case between the two forces in Afghanistan. British officials said they were investigating the error, which comes amid growing concerns about civilian deaths from U.S. airstrikes.

The troops were patrolling northwest of Kajaki, a militant hotspot in southern Helmand province, when they were attacked by Taliban fighters, Britain’s Ministry of Defense said in a statement.

“During the intense engagement that ensued, close air support was called in from two U.S. F15 aircraft to repel the enemy. One bomb was dropped and it is believed the explosion killed the three soldiers,” it said.

In Washington, a Pentagon official said initial reports were that the airstrike was called in by a British forward air controller. The forward controller is usually the person on the ground, who has the target area in sight and directs an aircraft to attack, giving target coordinates and ensuring that friendly forces are not in the way.

The incident has to be carefully investigated to try to learn where the problem arose, the official said on condition of anonymity because he was not authorized to speak on the record. That is, officials have to try to determine whether human error, either by the troops calling in the coordinates for the airstrike or by pilots dropping the bomb, was behind the accident. They will also need to examine whether equipment failure, either in the ground or in the air, was a factor.

“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,” said a spokesman for British troops in Helmand, Lt. Col. Charlie Mayo.

British Defense Secretary Des Browne declined to speculate on the cause of the friendly fire, which took place about 6:30 p.m. But he said he did not want “to get into a situation where we are blaming each other.”

“As a matter of fact, U.S. air support has saved our people’s lives on many, many occasions, particularly over the last four months in that very theater,” he told Sky News. “The nature of this war-fighting is such that there are comparatively small margins of error involved.”

The British solders were traveling through typical Afghan terrain – semi-agricultural, with many ditches and walled compounds where insurgents can hide – when they came under attack, the British army said. It was unclear whether the soldiers were on foot or traveling in a vehicle.

Kurt Volker, principal deputy assistant secretary of state for European and Eurasian affairs, said the United States was deeply saddened by the deaths.

“We offer our heartfelt condolences to the families and loved ones of the soldiers who died, and we wish those who were injured a speedy recovery,” he said in a statement. “We will investigate this tragic incident thoroughly with our British allies. We are committed to making information available as quickly as possible.”