It is astonishing to discover that the same small American unit, the US Marine Corps' Special Operations or MarSOC, has been responsible for all three of the worst incidents in Afghanistan in which civilians have been killed. Its members refer to themselves as "Taskforce Violence" and the Marines' own newspaper scathingly refers to the unit as "cowboys".

The US military commanders in Afghanistan must have known about MarSOC's reputation for disregarding the loss of life among Afghan civilians, yet for 10 days, they have flatly denied claims by villagers in the western Afghan province of Farah that more than 100 of their neighbours had been slaughtered by US air strikes.

Everything the US military has said about the air strikes on the three villages in Bala Boluk district on the evening of 4 May should be treated with suspicion – most probably hastily-concocted lies aimed at providing a cover story to conceal what really happened. Official mendacity of these proportions is comparable to anything that happened in Vietnam.

The US military now seem to have dropped their previous suggestion that Taliban gunmen had run through the village streets lobbing grenades into houses because villagers had failed to give them a cut of the profits from the opium crop. No evidence was produced for this unlikely tale. Witnesses saw no signs of grenade blasts or machine gun fire. A US official source in Washington eventually admitted that the claim was "thinly sourced".

Survivors from Gerani, Gangabad and Khoujaha villages say that there had been fighting nearby but the Taliban had long withdrawn when US aircraft attacked. This was not a few errant sticks of bombs but a prolonged bombardment. It had a devastating effect on the mud-brick houses and photographs of the dead show that their bodies were quite literally torn apart by the blasts. This makes it difficult to be precise about the exact number killed, but the Afghan Rights Monitor, after extensive interviewing, says that at least 117 civilians were killed, including 26 women and 61 children.

The US military has now fallen back on the tired old justification that the enemy was using civilians as human shields. This certainly is not satisfying infuriated Afghans from demonstrating students at Kabul university all the way to President Hamid Karzai. Whatever MarSOC troops thought they were doing in Bala Boluk, the killing of so many civilians will do nothing but strengthen the Taliban.