Human Rights Watch has called on Nato-led forces in Afghanistan to release the results of an investigation into a March incident in which an eight-year-old girl was burned by white phosphorus munitions.
The girl, Razia, was badly burned when her home was hit by a shell containing the chemical, which ignites on contact with air and can stick to flesh. US military medical staff who treated the girl said she had white phosphorus on her face and neck.
Two of her sisters were killed in the strike in the eastern province of Kapisa. The case, the first known example of civilian casualties from white phosphorus in Afghanistan, was first reported by Reuters yesterday.
The chemical can be used legally in war to provide light, create smokescreens or burn buildings, so it is not banned under treaties that forbid using chemicals as weapons. But its use in populated areas has been a persistent source of controversy.
Razia's father said Western forces had fired the shot, part of a volley aimed at Taliban fighters. US and French Nato troops based in the area were battling insurgents at the time.
But US Major Jennifer Willis, a Nato spokeswoman, said an internal investigation had concluded it was "very unlikely" that rounds fired by Western forces had hit the house, and suggested mortar rounds fired by Taliban militants may have been to blame.
In a statement issued after Reuters reported the case, New York-based watchdog Human Rights Watch called on Nato to reveal the full contents of its probe.
"Nato has not denied using white phosphorus during the Kapisa incident, nor have they provided evidence that the insurgents fired these rounds," the group's senior military analyst Marc Garlasco said in the statement.
"Nato and US forces need to reassure the people of Afghanistan, already alarmed by high civilian casualties, that these munitions are not being used unlawfully," he said.
Western forces have acknowledged that they use white phosphorus in Afghanistan. Major Willis said the Taliban have used it too, giving details of four incidents over the past 18 months in which NATO troops reported either coming under fire from white phosphorus rounds or discovering them in the field.
But Mr Garlasco told Reuters those "seem like isolated incidents compared to the widespread and regular use of white phosphorus by US and NATO forces".
"Human Rights Watch believes that the use of white phosphorus munitions in densely populated areas violates the requirement under international humanitarian law to take all feasible precautions to avoid civilian injury and loss of life," the group said in its statement.