The continued closure of Pakistani supply routes to U.S. and coalition forces in Afghanistan is costing the Pentagon nearly $100 million a month, Defense Secretary Leon Panetta told Congress on Wednesday.

American military planners have been forced to use supply lines in Central Asia, known as the Northern Distribution Network, to move men and materiel to and from Afghanistan since last November, after an errant U.S. airstrike ended in the deaths of Pakistani troops.

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Transiting troops and equipment out of Afghanistan through Central Asia has been an extremely pricey endeavor, compared to using the cheaper, more direct land routes through Pakistan, Panetta told members of the Senate Appropriations Committee.

As a result, the Pentagon is losing "$100 million a month because of the closure of those [ground lines]" in Pakistan, Panetta said.

Washington and Islamabad were reportedly close to a deal in May to reopen the lines, which have been closed to American and NATO forces since Pakistan shut the routes in retaliation for the airstrike deaths.

But Pakistan's decision to increase the price-per-truck cost to the United States and its allies to move supplies through the country eventually scuttled the deal.

Islamabad also continues to demand a formal administration apology for the incident.

U.S.-Pakistani talks on the issue have since been suspended and the U.S. negotiators were pulled from the country on Monday.

During Wednesday's hearing, Sen. Dianne Feinstein Dianne Emiel FeinsteinThis week: Senate kicks off Supreme Court fight Senate Democrats want to avoid Kavanaugh 2.0 Conservative group unveils ad accusing liberals of attacking Barrett's faith MORE (D-Calif.) pressed Panetta on whether an official White House apology would be enough to break the impasse.

"The apology is all-important ... and I think the position is that the national security of this nation is best served if we can develop a positive relationship with Pakistan," Feinstein said.

But recent comments by Panetta, chastising Pakistan for providing terror groups such as al Qaeda and the Haqqani Network safe haven inside the country, have only hardened Islamabad's position on the supply routes.

“It is an increasing concern that the safe haven exists and that there are those — likely Haqqanis — who are making use of that to attack our forces," Panetta said Thursday during a joint press conference with Afghan Defense Minister Abdul Rahim Wardak.

"We are reaching the limits of our patience here," he said at the time.

On Wednesday, Panetta steered clear of any similar condemnations of Pakistan's ties to the Haqqanis or other terror groups.

He also noted that the United States has already "expressed the condolences for the mistakes we have made" in Pakistan, both privately and publicly.

"I think the problem is that at this point ... there are other elements to the negotiation that are also involved that have to be resolved," Panetta said. "So [the apology] alone ... isn't the only issue that's being discussed and that needs to be resolved in order to get the [routes] open."