It's been almost two months since Pakistan closed the two key border crossings into Afghanistan used to supply NATO troops in Afghanistan, and though alternate routes have kept the supplies flowing, the financial cost has been substantial.

A Pentagon official says the cost of moving supplies into Afghanistan is now $104 million a month.  That's $87 million more than the $17 million it used to cost to transport supplies when the border crossings were open.

The cost estimate includes the added costs of the combined ground and air movements being used to offset the closed border crossings.

The 512 percent increase in monthly costs resulted from Pakistan's shut down of the border crossings at Torkham and Chaman shortly after a NATO air strike in late November mistakenly killed 24 Pakistani soldiers serving at a remote border outpost.  The Pakistani government closed the crossings to show their displeasure with the U.S. and NATO in the wake of the deadly attack.

Pentagon officials have said that the closure of the border crossings has not impacted NATO's military operations inside Afghanistan.

But rerouting those supplies has proven costly.

Though the overland routes through Pakistan were a crucial entry way for fuel supplies, in recent years U.S. military planners had expanded another ground supply route known as the Northern Distribution Network (NDN).

Coursing its way through Russia and the former Soviet republics that border Afghanistan, the seven supply routes that form the NDN were already the main entry point for non-lethal supplies for NATO in Afghanistan.

When Pakistan closed the border crossings, only 30 percent of NATO supplies flowed through them, most of it fuel.

A Defense official says most of the added costs come from the diversion of supplies originally intended to go through Pakistan that now arrive by ship in other countries in the region for eventual air transport into Afghanistan.

For example, there is the added cost in diverting some cargo from Pakistani ports to Indian ports where the supplies are either flown into Afghanistan or transported northward by train for delivery through one of the NDN routes.

Additional costs come from the transportation of more materials through the NDN, and the even pricier cost of flowing in supplies on direct flights from the U.S. or Europe into Afghanistan.

The direct flights cost ten times what it would cost to transport materials through Pakistan.  That is one of the reasons it is the least used option.

Transporting materials through the NDN is estimated to be three times the cost of transporting supplies through Pakistan.

Pakistan has not indicated that it will reopen the border crossings any time soon which could mean that using the costly alternate routes will be the only options for the foreseeable future.