As if tensions weren't high enough between the United States and Pakistan, new internal Pentagon documents reviewed by the Wall Street Journal reveal that the U.S. has been denying 40 percent of Pakistan's expense claims.

Washington pays Islamabad billions of dollars to fight al Qaeda and other militants, but after expressions of mistrust by many in Congress, the U.S. decided that the claims were exaggerated, or of little use in the war on terror.

For example, the Pakistani army billed the U.S. $50 million for hygiene & chemical expenses, of which the U.S. agreed to pay only $8 million, according to records covering January 2009 through June 2010, the Journal wrote. Pakistan's Joint Staff - the country's top military brass-requested $580,000 in 2009 to cover food, medical services, vehicle repair and other expenses, but the U.S. paid nothing.

According to the Pentagon documents, Pakistan billed the U.S. $3.2 billion; and the U.S. denied $1.3 billion of that sum.

In another instance, the U.S. paid millions to refurbish four helicopters expected to be used to transport troops to fight against the Taliban, but the aircraft was routed to peacekeeping duties in Sudan - operations which the United Nations funds for Islamabad.

This is about how much money Pakistan can extract, said Moeed Yusuf, South Asia adviser for the United States Institute of Peace, and independent research organization funded by Congress.

Meanwhile, Pakistani officials deny they are taking advantage of the U.S.

People have to give a receipt for every cup of tea they drink or every kilometer they drive, one Pakistani official said.

U.S. officials say Pakistan's claims have been rejected for a number of reasons.