Failure by US forces to follow their own rules was the "likely" cause of civilian deaths in Afghan airstrikes last month, a US military report says.

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US officials investigated seven strikes on Taliban targets in Farah province on 4 May, and concluded that three had not complied with military guidelines.

The report accepts that at least 26 civilians died, but acknowledges that the real figure could be much higher.

The Afghan government has said 140 civilians were killed in the strikes.

Washington and Kabul have been at loggerheads for weeks over the number of civilians killed in the incident.

The US report defends the Farah operation, saying the use of force "was an appropriate means to destroy that enemy threat".

"However, the inability to discern the presence of civilians and avoid and/or minimise accompanying collateral damage resulted in the unintended consequence of civilian casualties," the report says.

It says the final three strikes of the engagement, which took place after dark, did not adhere to "specific guidance" in the controlling directive.

"Not applying all of that guidance likely resulted in civilian casualties," the report says.

It concedes that the precise number of civilians killed in the attack may never be known because many victims were buried before the investigation started.

The document makes a number of recommendations to reduce the likelihood of civilian deaths.

It says lines of communication must be improved, new guidelines should be introduced and personnel need to be retrained.

Gen Stanley McChrystal, the US commander in Afghanistan, is currently reviewing US rules in relation to airstrikes.

He said last month that US forces should use them only if the lives of Nato personnel or American troops were clearly at risk.

Both Nato and US have have insisted that avoiding civilian casualties is their priority in all battles.