The U.S. military plans to taper off its airdrops of food in Afghanistan as convoys reach more areas by land, a Pentagon official said Thursday.

The collapse of Taliban control in large swaths of Afghanistan has greatly improved aid groups' ability to help suffering Afghans, said Joseph J. Collins, a humanitarian assistance planner at the Defense Department.

He called the Taliban the biggest obstacle to effective humanitarian programs in Afghanistan, accusing the militia of hijacking aid shipments, beating humanitarian workers and putting troops in vehicles with United Nations markings.

The U.N.'s World Food Program and other aid groups dramatically increased their land shipments of food to Afghanistan in the past six weeks, even during American airstrikes, Collins said. So far, the United States has dropped about 1.5 million humanitarian food packets in Afghanistan, mostly in areas controlled by anti-Taliban forces, he said.

Most of the convoys traveled without armed escorts, which means future food convoys might not need troops to protect them, he said.

"Food transport is not really a question of security," Collins said. "It was done during the heaviest fighting."

Instead of airdrops, the U.S. military's role in humanitarian aid will shift to helping the U.N. and aid groups transport food and other supplies to Afghanistan, Collins said. The U.S. military also may provide expertise and security to help rebuild roads and airports in Afghanistan needed for aid shipments, he said.

Collins said there is enough time before the onset of winter to provide food and other supplies to the Afghans that need them.

"The winter problem is exaggerated in some cases," he said. "It's a real problem in the central part of the country and parts of the northeast, but most areas are passable during winter."