A video has surfaced online that appears to show a US helicopter crew singing "Bye-bye Miss American Pie" before blasting a group of Afghan men with a Hellfire missile.

The footage comes in the wake of a string of damaging videos and pictures showing US forces in Afghanistan urinating on the bodies of dead insurgents, and posing with the remains of suicide bombers and civilians killed for sport by a group of rogue soldiers.

If it is proved to be authentic, it could further undermine the image of foreign forces in a country where there is already deep resentment owing to civilian deaths and a perception among many Afghans that US troops lack respect for Afghan culture and people.

The posting says the video was recorded in Wardak province, which lies south-west of the capital, Kabul, in September 2009. The caption refers sarcastically to a group of "innocent farmers planting poppy seeds in the middle of the road".

Men spotted digging in Afghan roads by the US or other foreign forces are likely to fall under suspicion that they are insurgents burying home-made bombs, one of the Taliban's main weapons.

If the US military is confident it has identified them as insurgents, bombs are sometimes used to kill them, although Afghan officials have accused troops in the past of killing farmers and people working on irrigation ditches when they thought they were targeting people laying bombs.

In the video, after the bomb appears to hit the group, survivors scatter, and the helicopter aims machine gun fire at them.

"We're aware of the video that was posted that appears to be a recording of an Isaf aircraft engagement in Wardak," said Martin Crighton, a spokesman for the Nato-led International Security Assistance Force (Isaf) in Afghanistan.

"Unfortunately the video does not appear to have a date stamp on it to allow us to put it in an immediate context. However any impression of impropriety on the video is not representative of the professional service members who make up the Isaf coalition."

In April, the Los Angeles Times published pictures that appeared to show American soldiers posing with the bodies of dead Afghans in the south of the country, and the US president, Barack Obama, called for an investigation.

That came after several damaging months for the US military in Afghanistan that heavily undermined trust in their conduct and motives.

In March a US soldier killed 16 civilians on a solo night-time shooting rampage. Deadly violence erupted in February over the burning of copies of the Qur'an by US troops. In January a video surfaced of marines apparently urinating on Taliban corpses, and last year a group were tried for murdering three Afghan civilians for sport.