A Pentagon audit has found that a British contractor in Afghanistan billed the US government more than $50 million for questionable items, including seven luxury cars, US Senator Claire McCaskill said.

McCaskill wrote a letter to Secretary of Defense James Mattis demanding answers on the "Legacy East" contract, a project to provide counter-insurgency intelligence experts to mentor and train the Afghan National Security Forces.

In her letter, McCaskill noted that a recent Defense Contract Audit Agency (DCAA) review of the contract had called more than $50 million in expenses into question. The audit found that a subcontractor, New Century Consulting, billed over $50 million in questionable costs to the Army through its contractor, Imperatis.

"US Senator Claire McCaskill, the top-ranking Democrat on the Senate Homeland Security and Governmental Affairs Committee, today blew the whistle on a federal contract that left taxpayers on the hook for over $50 million in questionable costs, including seven luxury vehicles and $400,000 average salaries for significant others of corporate officers to serve as 'executive assistants,'" McCaskill's office said in a statement on Wednesday.

The items paid for included seven luxury cars — including Porsches, Alfa Romeos, a Bentley, an Aston Martin and a Land Rover, McCaskill said.

The audit also found that the significant others of the CEO and CFO were paid salaries of approximately $420,000 each to work as "executive assistants," though they worked from home and no documentation exists to prove they actually performed any work, McCaskill said.

The company also spent over $1,500 on alcohol and $42,000 in cash on automatic weapons, though the contract provisions prohibit such expenditures.

Other questionable expenses included severance payments, rent, unnecessary licensing fees, extensive austerity pay and expenses for air travel for personal reasons, McCaskill said.

The senator demanded that Mattis say who at the Defense Department managed the contract and explain what steps the Pentagon was taking to recover the questioned costs.