AFP - A failure by US forces to follow procedures in deadly air strikes last month in Afghanistan "likely" caused the death of at least 26 civilians, the US military said on Friday.

During a May 4 battle with Taliban insurgents in western Afghanistan, US air crews and ground troops acted in line with the laws of armed conflict, but three air strikes by a B-1 bomber "did not adhere to all of the specific guidance" under US combat rules and orders, a military investigation concluded.

"Not applying all of that guidance likely resulted in civilian casualties," said a summary of the probe released Friday.

The investigation found that about 26 civilians died in the incident but said it was possible that a higher number were killed.

The Afghan government has put the civilian toll for the incident at 140.

The investigators wrote that "no one will ever be able conclusively to determine the number of civilian casualties that occurred on May 4, 2009."

The report described an intense battle with Afghan security forces calling for assistance from a team of US Marines near the village of Gerani in the Farah province.

Trying to help Afghan forces pinned by the insurgent gunfire and secure the evacuation of wounded soldiers, the Marine unit called in air strikes.

A first wave of strikes by F/A-18 fighter jets hit intended targets without causing civilian casualties, according to the report.

After nightfall, a B-1 Lancer bomber was called in to relieve the F/A-18s and carried out three strikes using 500-pound and 2,000-pound bombs.

In two of the three raids, the bombing likely caused civilian deaths, the report said.

In both cases, air crews and the ground commander believed they were targeting insurgent forces that were still firing on Afghan government and coalition forces.

But neither the ground force commander or the bomber crew "could confirm the presence or absence of civilians already in the building," it said.

In the third strike, the crew of the bomber and ground force commander spotted a group of adults that they suspected were Taliban forces "moving rapidly in the dark across difficult terrain in an evenly-spaced formation."

The ground commander ordered the B-1 bomber to hit a building where he believed the insurgents had moved.

The aircraft then dropped a 2,000-pound bomb on the building, heavily damaging it and a nearby structure.

In a series of recommendations, the investigation said it was vital for the US mission in Afghanistan to adopt tactics that put a priority on avoiding civilian casualties.

The probe said that US and coalition forces should review and refine all combat rules, including the use of air power, for situations where there is a risk of civilian casualties.

Units in the country and due to deploy need to carry out immediate training once the new guidance is in place, it said.

The probe also called for better public relations efforts in cooperation with Afghan ministries, better communication with leaders of local non-governmental organizations and the creation of investigative teams that can quickly respond to reports of possible civilian casualties.

US officials have grown increasingly concerned about the effect of civilian casualties on the war effort, amid growing public anger among Afghans and tensions with the Kabul government over the issue.