A box of leaflets dropped by an RAF plane in Afghanistan landed on and killed a young girl, the Ministry of Defence said.

The box should have broken apart in mid-air but struck the young girl intact. She was taken to a hospital in Kandahar where she died. The leaflets were dropped over a rural area of Helmand province by an RAF C130 Hercules on June 23.

"It is a matter of deep regret that one of the boxes failed to fully open and on landing caused serious injuries to an Afghan child," said a spokesman.

"The child was treated at a local medical facility in Kandahar where, despite the best efforts of staff, she died as a result of her injuries. An investigation into the incident is under way."

Defence sources said the girl was taken to a local hospital in Kandahar instead of to an International Security Assistance Force hospital, which may have been better equipped.

The MoD would not comment on what type of leaflet was involved but said the air drops were common and in the past had included public information on the presidential elections and basic warnings about explosives.

The news of the death came as Gordon Brown told Sky News that where the military wants new equipment "they have got it" and refused to rule out sending more troops to Afghanistan.

"I can say that we will do whatever is necessary and that does not rule out doing what is necessary for our troops," Brown said.

"Where they want new equipment – and there have been a thousand new vehicles gone into the defence department costing a billion pounds – where they need refurbishment or new helicopters, where they need new equipment like night vision equipment, they have got it.

"I defy you to say that any urgent operational requirement that is needed by the military has been turned down by the Treasury."

A UN report published on Saturday said August was the deadliest month of the year for civilians in Afghanistan because of violence from the insurgency. About 1,500 civilians died from January through August this year, up from 1,145 for the same period of 2008.