Defense Secretary Leon Panetta said today that Pakistan's closure of NATO's ground supply routes into Afghanistan is costing the U.S. $100 million a month as supplies have been rerouted elsewhere.

It is the first time in six months that an official dollar amount has been provided for how much the rerouting has cost. In January, Pentagon figures obtained by ABC News said the rerouting of supplies through the "Ground lines of communication" (GLOC's) was costing $104 million a month, a huge increase over the pre-closure monthly cost of $17 million. That initial cost now appears to have remained virtually unchanged since then.

Today Panetta told a Senate Appropriations subcommittee, "We've also had the closure of these ground lines - the so-called GLOCs - in Pakistan, and the result of that is that it's very expensive because we're using the northern transit route in order to be able to draw down our forces and, also, supply our forces. I think the amount is about $100 million…a month."

Pakistan closed the ground supply routes into Afghanistan shortly after a deadly border incident, in which U.S. forces, thinking they were up against enemy forces, killed 24 Pakistani soldiers on November 26.

The U.S. has been trying for months to reopen the GLOC's without much success. A U.S. negotiating team left Pakistan this week without a deal after an apparent stalemate in the talks.

Before Pakistan closed the border crossings, 30 percent of NATO supplies flowed through them, mostly fuel.

Pakistan has demanded a U.S. apology for the November attack, but the U.S. has only offered an expression of regret. It has also been reported that the Pakistanis have wanted to charge $5,000 per truck that goes through its territory into Afghanistan, a significant increase over the $250 that had been charged before the closure of the GLOCs.

The increase in costs comes because the U.S. has had to re-route supplies through countries that border northern Afghanistan, what's known as the Northern Distribution Network (NDN).

Publicly, U.S. officials have said the rerouting hasn't had much of an impact on supplies for NATO troops in Afghanistan. Privately, Defense officials acknowledge that the routes through Pakistan are actually more important for the future removal of equipment from Afghanistan, slated for the end of 2014.

After Panetta's remarks today a Defense Department official said that the $100 million figure he was citing was actually $100 million more a month than the Pakistan routes used to cost. In total, the latest government figures show the war in Afghanistan costs the U.S. $6 billion a month.