A failure by US forces to follow procedures in air strikes last month in Afghanistan "likely" caused the death of at least 26 civilians, according to the US military.

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 on Friday.

The investigation found that 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 at 140 for the incident, which has fueled public anger and led President Hamid Karzai to demand a halt in air strikes.

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 recommended tactics and combat rules be reviewed and refined to avoid civilian casualties but did not call for prosecution of any US military personnel.

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 down by the insurgent gunfire and secure the evacuation of wounded soldiers, the Marine unit called in air power.

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.

2,000-pound bomb dropped on building

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.

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

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

The probe called for combat rules - including the use of air power - for US and coalition forces to be reviewed and revised for situations when civilian lives are at risk.

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

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

It said leaders of NGOs, such as the International Committee of the Red Cross or an Afghan human rights group, need to be able to reach military commanders in the Nato-led force "on very short notice."

And coalition officers need to regularly review military operations with NGO leaders, it said.