Washington  - A Pentagon investigation into a US airstrike in Afghanistan last month that killed civilians has concluded there were problems with how the attack was carried out, but said it was "lawful."

Investigators estimated that the number of civilians killed during the May 4 airstrike in Afghanistan's Farah province was 26. However, the unclassified version of the report released Friday by US Central Command revealed that the exact number of civilian casualties may never be known.

The Afghan government said the strikes left at least 100 civilians dead.

"We will never be able to determine precisely how many civilian casualties resulted from this operation," the report said, "but it is inconsistent with the US government's objective of providing security for the Afghan people to conduct operations that result in their death or wounding."

At least 78 Taliban fighters were killed in the attack.

The report acknowledged that while the airstrike was "lawful," several standards specified in operational guidance were not met. "Not applying all of that guidance likely resulted in civilian casualties," the report said.

On June 8, Pentagon spokesman Geoff Morrell conceded there were "problems" with how the attack was carried out.

"There were some problems with some tactics, techniques and procedures or ... the way in which close air support was supposed to have been executed in this case," he said.

US airstrikes have frequently killed Afghan civilians and been a source of tension between Washington and Kabul. Days after the Farah attack, US Defence Secretary Robert Gates expressed regret for the loss of innocent life but accused the Taliban of using noncombatants as human shields. (dpa)