At least 30 civilians were killed when the US bombed several drug-making facilities in western Afghanistan in May, a UN agency said in a report yesterday, though the US military immediately disputed the findings.

The United Nations Assistance Mission in Afghanistan (UNAMA) conducted an investigation over four months looking into what happened May 5 when the US military bombed dozens of sites it had identified as Taliban methamphetamine labs.

Soon after the strikes in Bakwa district of Farah province, and parts of the bordering Delaram district in Nimroz province, UNAMA said it began to receive reports of "significant civilian harm".

After a fact-finding mission to some of the impact sites and face-to-face interviews with 21 people impacted by the strikes, UNAMA said it had "verified 39 civilian casualties (30 deaths, five injured and four undetermined), including 14 children and one woman, due to the 5 May air strikes".

The agency went on to say that it had also received "credible information" about an additional 30 deaths -- mostly women and children - and was working to further verify these claims.

US Forces-Afghanistan (USFOR-A) blasted the UNAMA findings, questioned the agency's methodology and insisted its “precision" strikes had accurately targeted meth labs.

"In addition to imagery collection during the precision strikes, USFOR-A conducted exhaustive assessments of the facilities and surrounding areas after the strikes," the command said in a statement.