KABUL, Afghanistan – An American 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 American troops has led to the deaths of three British servicemen in the current Iraq war, but the incident last 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 American 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 American 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, Lieutenant Colonel 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 America 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.”

Britain has about 7,000 troops in Afghanistan, most based in Helmand, the world’s leading supplier of opium. The troops have been battling militants for months in Kajaki, where repairs are taking place on a hydroelectric dam that could supply close to 2 million Afghans with electricity.

The deaths bring to 73 the number of British personnel killed in the country since the American-led invasion in November 2001.

Taliban insurgents in the east and south of the country have stepped up their attacks on Afghan and coalition forces over the last 18 months, seeking to overthrow the Western-backed government installed after the ouster of the Taliban.

Afghanistan’s rugged terrain and poor roads, as well as a shortage of ground troops, have forced the United States to rely heavily on airpower in the fight against Taliban and other militants. Airstrikes have led to the deaths of hundreds of civilians over the last two years, undermining efforts to win the trust of Afghans – key to victory in any insurgency, experts say.

Sir Sherard Cowper-Coles, Britain’s ambassador in Kabul, told The Associated Press last week that “occasional tensions” inevitably surface where forces from different nations are fighting alongside one another. Some of those tensions involve the deaths of Afghan civilians.

Yesterday’s bombing was not the first case of friendly fire in Afghanistan.

In 2002, four Canadian soldiers were killed when an American F-16 pilot on a night patrol dropped a 500-pound bomb on Canadian troops conducting a live-fire training exercise near the southern city of Kandahar. The pilot apparently mistook the Canadians for enemy forces and thought he was acting in self-defense, American officials have said.

In August 2006, a bomb mistakenly dropped by coalition aircraft killed 10 Afghan police officers on a patrol in the country’s southeast. The cause of that incident has never been established.

In the most famous friendly fire case of the Afghan conflict, Pat Tillman, the former NFL player who became an Army Ranger, was killed in April 2004 by fellow troops near the Pakistani border.

In Iraq, friendly fire incidents have been blamed on equipment failure or human error.

After an inquest into the death of British soldier Lance Corporal Matty Hull, 25, killed in a friendly fire attack by two American pilots in Iraq in 2003, opposition legislators in Britain called for improvements in joint identification systems.

Britain last year threatened to end cooperation with America on the new Joint Strike Fighter jet after 10 years of development, until the Pentagon resolved concerns it was not sharing enough information about the aircraft’s sensitive software with London.

Friendly fire involving American troops in Iraq have led to the deaths of 12 British servicemen since 1990.

In the ground battle to drive Iraqi forces from Kuwait in the 1991 Gulf War, nine British soldiers were killed when an American Air Force A-10 “tank buster” mistakenly fired on two British armored vehicles in a battle between British and Iraqi troops. The A-10, a slow-flying jet designed to destroy tanks, was providing air support for the British and mistakenly hit their armored vehicles while attacking an enemy position.

___

Associated Press writers Noor Khan in Kandahar, Fisnik Abrashi in Kabul, Pauline Jelinek in Washington and Jill Lawless in London contributed to this report.