Image caption The US military report follows a six-month investigation

Investigators say the US military has been giving tens of millions of dollars to Afghan security firms who are channelling the money to warlords.

Trucks carrying supplies to US troops allegedly pay the firms to ensure their safe passage in dangerous areas of Afghanistan.

The convoys are attacked if payments are not made, according to allegations in a US military document.

The congressional report follows a six-month investigation.

The document states that trucks carrying food, water, fuel, and ammunition may be supplying up to $4 million (£2.7m) per week to the firms.

A US congressional committee is expected to hear the evidence on the investigation from senior officials at the US Department of Defense later on Tuesday.

'Vast protection racket'

The congressional subcommittee that carried out the investigation says that bribes are paid to the Taliban and virtually every governor, police chief and local military commander whose territory the convoys pass through.

One of the security companies in question is alleged to be owned by two cousins of the Afghan President Hamid Karzai.

The report released late on Monday says the security agreements violate laws on the use of private contractors, as well as US Department of Defense regulations.

The report states that "although the warlords do provide guards and coordinate security, the contractors have little choice but to use them in what amounts to a vast protection racket".

The document states that security companies hired under the Afghan Host Nation Trucking are the ones funneling the money.

Watan Risk management is one of the largest security providers in Afghanistan and one of the companies currently under investigation.

The military report states that representatives of the company allegedly negotiate or dictate the price of safe passage in a given area. The company then issues warnings to trucking companies who are late in paying or refuse to pay the sum.

A spokesman for the Army Criminal Investigation Command at Fort Belvoir, Virginia, confirmed to the AP news agency that the inquiry is taking place.

The report comes as the number of US casualties is rising in Afghanistan, and suggests not only that money from the US tax payer is being used to finance the enemy, but also to undermine international efforts towards stability in the country.