From June through August, U.S. troops detected or were hit by 5,088 improvised explosive devices (IEDs), the most for any three-month period since the war began in 2001.
Those bombs killed 63 troops and wounded 1,234, Defense Department records show.
More than 80% of the IEDs are homemade explosives using calcium ammonium nitrate fertilizer produced in Pakistan, said Navy Capt. Douglas Borrebach, deputy director for resources and requirements at the Pentagon's Joint IED Defeat Organization.
"The border is a sieve," Borrebach said. "You can do your checkpoints, but that's not going to help stem the supply."
The military is working with the State Department, other U.S. agencies and Pakistan's government to prevent fertilizer from reaching the insurgents' bomb factories.
U.S. troop casualties from improvised explosive devices in Afghanistan*:
* June through August; Source: U.S. Defense Department
The U.S. government increasingly has been blaming Pakistan for failing to corral insurgents. Two weeks ago, Adm. Michael Mullen, then the chairman of the Joint Chiefs of Staff, said the Haqqani terror network linked to attacks in Afghanistan had ties to Pakistan's spy agency.
A Senate bill includes funds to train border guards and customs officials in Pakistan and Afghanistan. It also supports agricultural extension programs that encourage Pakistani farmers to use alternative fertilizers.
Sen. Robert Casey, D-Pa., met with top civilian and military leaders in Pakistan in August to urge them to stop the flow of bomb-making materials into Afghanistan. They promised to help, but did not reveal a sense of urgency, Casey told USA TODAY after the trip.
Choking off the source of fertilizer is critical, Borrebach said. "How do we work with Pakistan to be able to reduce the amount of calcium ammonium nitrate coming across the border?" he said. "That's the key to this."
Not necessarily, said Seth Jones, an expert on Afghanistan at the RAND Corp. who has advised the special operations forces there. "You could bang your head against a wall for eternity trying to keep ammonium nitrate from crossing the border," Jones said.
The Taliban and other insurgent groups operate out of a border region between Pakistan and Afghanistan, over which the Pakistani government has little control.
Beefing up local security forces in Afghanistan, he said, shows more promise in defeating the IED problem. He pointed to areas of Kandahar province where homegrown security forces, assisted by the U.S. troops, had made life better for local citizens. They, in turn, rejected insurgents, sided with the security forces and pointed out bomb caches. The IED problems subsided, he said.