WASHINGTON - American and coalition forces in Afghanistan will be more vulnerable to improvised explosive devices as the military draws down troops next year, a senior Pentagon official said Thursday.

Lt. Gen. Michael Barbero described his concerns about what is the top cause of military and civilian deaths in Afghanistan and Pakistan in congressional testimony that also underscored U.S. frustration with Islamabad's efforts to thwart the production of the devices known as IEDs, most of which are fertilizer-based explosives.

IEDs are responsible for more than 60 percent of U.S. troop deaths and injuries in Afghanistan as the war has entered its second decade. Although the number of incidents is down this year, IEDs caused 1,874 American casualties.

Barbero told a Senate Foreign Relations subcommittee that the drawdown of about 66,000 U.S. troops next year will make forces more susceptible to IEDs. He said fewer troops will mean travel will be more predictable, raising the possibility of more attacks. In addition, fewer troops will mean less awareness of what's happening.

"IEDs will continue to be the weapon of choice against our forces," Barbero told the panel.

Barbero, director of the Joint Improvised Explosive Device Defeat Organization, and Jonathan Carpenter, a senior economic adviser at the State Department, insisted that Pakistan, which has had more than 926 IED attacks and more than 3,700 casualties, needs to do more to stop the devices.

About 70 percent of the homemade explosives are made with ammonium nitrate from calcium ammonium nitrate. The agricultural fertilizer is produced by two factories in Pakistan.

Barbero said the Fatima Group, which owns and operates the two factories, has not been cooperative. In addition, the Pakistan government stopped direct communication between the United States and the company.

Sen. Bob Casey (D., Pa.), chairman of the subcommittee, said Pakistan is vital to stopping key components from reaching Afghanistan. "I see too many casualties at Walter Reed," Casey said of the military hospital. "We need to see action."