Image caption Improvised bombs planted by the Taliban have killed hundreds of foreign troops

Nato forces in Afghanistan have been exposed to greater risk of Taliban attacks because of fraud, a US government agency has alleged.

A report said Afghan contractors paid to seal drainage channels - often used to hide explosives under roads - had failed to do so in more than 100 cases.

A criminal investigation is under way, senior generals have been told.

The BBC's Andrew North in Kabul says if proven, the case is likely to deepen mistrust between Nato and Afghans.

In a letter released on Thursday, special inspector general for Afghan reconstruction John Sopko warned senior commanders that an Afghan contractor paid to seal drainage systems along strategic roads had left the work undone or badly done.

The Taliban have often used drainage channels to hide improvised explosive devices (IEDs).

Hundreds of foreign troops have been killed by such weapons since the war began in 2001.

To reduce the risk, the US military has paid contractors to install metal grates over the channels to prevent anyone getting inside.

But investigators said there was evidence of fraud in the work and in many cases no metal grate was installed at all.

"Given the increased risk of IED attacks against US forces resulting from missing or defective... systems, we are providing this information to you for immediate action and dissemination to all relevant personnel," Mr Sopko said in his letter to the top commander in Afghanistan Gen John Allen and Gen James N Mattis, head of the US Central Command.

Philip J LaVelle, a spokesman for the inspector general's office, declined to comment on whether any troops had been killed by the problem with the drainage channels.