The United States will order all its troops in Afghanistan to undergo new training after concluding that pilots violated orders in air strikes last month that it accepts may have killed as many as 86 civilians.
In a long-awaited report, released six weeks after US B1 bombers killed large numbers of civilians unleashing fury among Afghans, the Pentagon acknowledged that rules had not been followed, although it said the mistakes fell short of breaking the law.
The bombings took place on 4 May in western Afghanistan after a day-long battle that saw Afghan security forces ambushed by Taliban fighters and US Marines come to their aid.
After nightfall, B1 bombers observed groups of people moving into two houses and a mosque. Pilots concluded they were fighters and bombed the buildings.
However, the report said pilots broke guidelines by striking without checking whether civilians were in the buildings.
The strikes, "while complying with the (laws of armed conflict) did not adhere to all of the specific guidance and Commander's Intent contained in the controlling directive," it said.
"Not applying all of that guidance likely resulted in civilian casualties."
While the report noted that US investigators had concluded that about 26 civilians and about 76 fighters had died, it acknowledged the figures were imprecise and said the true civilian death toll would never be known.
But in the military's first public acknowledgement of Afghan accounts of much larger civilian tolls, the report noted that an Afghan human rights agency had concluded that 86 civilians had died and praised its findings as "balanced" and "thorough".
The report, released by Central Command responsible for the wars in both Afghanistan and Iraq, said the military needed to refine its rules for using weapons in Afghanistan, which should be published in new "stand-alone documents".
"Once this guidance is published, units will need to conduct immediate training/re-training of all personnel in theatre," it said.
The report supports accounts from Afghan villagers that the B1 bombings after dark occurred far from the battle zone.
One of the buildings, which the B1 flattened with two 2,000 lb (900 kg) bombs and two 500 lb bombs, was more than a kilometre away from where US and Afghan government troops were taking intermittent fire.
The report accepted that pilots and ground controllers believed the groups of people they were observing were fighters who might mass for an attack. But it acknowledged they were not firing from the buildings at the time they were struck.
While US rules of engagement are not public, a senior US military official in Kabul said this week they generally prohibit striking buildings unless steps are taking to ensure no civilians are inside, or troops on the ground are taking fire from them.
"You can see from the video, no one was firing (from the buildings). There was no imminent threat," the official said. "There needs to be an imminent threat."
The report made no mention of the Taliban deliberately using civilians as human shields in the incident, an assertion that became the main emphasis of US military statements about the bombings in the weeks after they took place.
The official acknowledged there was no firm evidence to prove the human shield allegation.
The bombings took place while Defense Secretary Robert Gates was on his way to Afghanistan to inform the commander of US forces that he would be replaced. The new commander, General Stanley McChrystal, has said he will take new steps to reduce civilian casualties, which threaten Afghan support for the war.
The report faulted the public response, saying US forces "must develop a more effective method" of communications.
It said the military needs to set up an investigative team headed by a general who could deploy to the scene of incidents involving suspected civilian casualties within two hours.
After the bombing last month, Afghan President Hamid Karzai went on US television and called for an end to air strikes.
The report said it did not recommend a halt to close air support, especially when friendly forces were under fire.
"However, absent a direct or imminent threat, we must pursue a tactical approach that prioritises avoidance of civilian casualties as a fundamental aspect of mission success."