A US warplane mistakenly bombed British troops after they called for air support in Afghanistan, killing three soldiers and wounding two others in an accident that could re-ignite debate about America's heavy use of air power. British officials said they were investigating the error, the first "friendly fire" incident between the two forces in Afghanistan. Similar mistakes by US troops in Iraq have led to the deaths of 12 British servicemen since 1990. The troops were patrolling northwest of Kajaki, a militant hotspot in southern Helmand province on Thursday evening 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 US F-15 aircraft to repel the enemy. One bomb was dropped and it is believed the explosion killed the three soldiers," it said.