Pakistan's army has deployed troops to a stretch of the Afghan border to stop Taliban militants fleeing a major U.S. offensive in southern Afghanistan, a spokesman said Thursday.

Nearly 4,000 U.S. Marines plus 650 Afghan forces moved into Afghanistan's Helmand province early Thursday to take on the Taliban in one of their strongholds.

Pakistani and US officials have expressed concern the American troop build up in southern Afghanistan could push the militants across the poorly guarded and mountainous border into Pakistan.

"We've mustered more troops from the other areas of the border" to deploy opposite the Helmand region, said Maj. Gen. Athar Abbas. "It is sort of a reorganization."

He said the threat of militants crossing over had been "visualized." He gave no more details.

Pakistan shares a 1,600-mile border with Afghanistan. The section opposite Helmand is around 160 miles long and lies in Baluchistan province, where U.S. officials believe the Taliban's top leadership are hiding out.

Washington wants Pakistan to crack down on militants based on its side of the border, blamed for launching attacks on US and NATO troops in Afghanistan. Pakistan has launched offensives in its northwestern border regions in recent months, drawing praise in the West.