To get to the source of the latest dispute between Afghanistan and the United States, you might have to travel down the road to Pakistan.
That's because Pakistan's northwest Khyber Pakhtunkhwa Province has been the scene of a weeklong NATO supply route blockade that has disrupted crucial military supplies heading for Afghanistan.
That interruption has helped fuel the latest diplomatic row between Kabul and Washington, in which the Afghan government has accused NATO of deliberately cutting vital supplies to the Afghan army and police. The alliance has rejected the allegations, but NATO's dependence on Pakistani supply routes suggests that the blockade is having a trickle-down effect.
Hundreds of demonstrators, demanding an end to U.S. drone strikes in Pakistan, have assaulted drivers and halted trucks bound for Afghanistan since November 23, preventing deliveries along one of two key routes. In addition, the blockade has prompted Washington to temporarily stop shipments of military equipment out of Afghanistan via Pakistan.
The hundreds of cargo trucks stranded on the border are essential to the fledgling 350,000-strong Afghan National Security Forces (ANSF), which includes the army and police, who still rely heavily on the Nato-led coalition in Afghanistan for fuel, vehicles, logistics, and other supplies.
Pakistani security forces have intervened to stop the demonstrations called by Khyber Pakhtunkhwa's newly elected government, led by former cricket star Imran Khan's Pakistan Movement for Justice (PTI). But transport unions say truckers have refused to resume their work for fear of being targeted, and Khan is claiming victory.
According to Yousaf Khan, president of the Union of the Transporters of Goods in Peshawar, protesters are stopping trucks in Peshawar and demanding that drivers prove they have no NATO goods in order to pass.
"The problem is that the protestors are so misguided that they stop each and every vehicle, including the one carrying cement without caring they are carrying goods for NATO or not," he says. "Ten to 20 people stop a vehicle and then check its documents and the vehicle keeps standing there."
Technically only the central government has the authority to determine the future of NATO supply routes on Pakistani soil. And while Islamabad has criticized U.S. drone attacks, calling them a violation of the country's sovereignty, it has shown no interest in repeating the seven-month blockade that followed a deadly U.S. airstrike near the Afghan border in November 2011.
The land routes through Pakistan have been crucial to getting supplies to NATO and Afghan forces in Afghanistan. The Khyber Pakhtunkhwa blockade, on a road that leads to the Torkhum border crossing, affects one of two key routes. The other is the Chaman border crossing in Pakistan's restive Balochistan Province.
Hanif Marwat, the president of All Pakistan Trailer Association, says companies may be forced to cancel all transport to Afghanistan if the blockade in Khyber Pakhtunkhwa continues.
"[Protestors] are breaking the vehicles' windshields, beating the drivers, and looting the goods," he says. "The owners can't do anything. We appeal to Prime Minister [Nawaz Sharif] that the PTI should carry out its demonstrations, but should not damage property."
The blockade may be the source of Afghan President Hamid Karzai's accusations this week that the United States could be withholding fuel and other key military supplies in an effort to pressure him to agree to a contentious security arrangement.
The president has refused to sign the Bilateral Security Agreement (BSA) with Washington despite earlier agreeing to the text of the proposed pact. His decision to delay its signing has infuriated the White House, which has threatened to pull out all American troops from Afghanistan after 2014 if Karzai does not sign the deal by December 31.
Waliullah Rahmani, the director of the Kabul-based Center for Strategic Studies, believes the spat over supplies has the potential to severely damage relations between Kabul and Washington, which are already under strain over the BSA.
"The incident has already fuelled tensions," Rahmani says. Now there is a damaging blame game going on. Whatever [the reasons for the alleged halt], such incidents will only inflame tensions between Washington and Kabul more and more."
Abdul Wahid Taqat, a retired Afghan general turned political analyst, notes a dangerous precedent -- the collapse of the Afghan communist regime in the early 1990s.
Back then, well-armed Afghan forces splintered after Moscow cut its funding and crucial fuel dried up, accelerating a brutal civil war that eventually brought the Taliban to power.
"If the United States has halted fuel supplies, it is an act of enmity against Afghanistan’s security establishment and forces," Taqat says.
A statement issued by the presidential palace on December 2 accused the United States of "applying pressure and creating dependency" by cutting fuel supplies to Afghan forces. It added that "cutting the supplies is not in line with U.S. commitments."
The statement cited information provided by the Afghan defense and interior ministers at a security council meeting on December 1. The ministers told Karzai that the interruption of fuel and equipment had occurred "two or three times" in recent days.
The coalition has denied any stoppage in the fuel delivery, saying: "We continue to process orders as soon as they are received from ANSF."
RFE/RL's Radio Mashaal contributed to this report