Pakistan has stepped up security along the Afghan border to stop any Taliban militants from crossing as they flee a major offensive by U.S. and Afghan forces.

A Pakistani army official says Pakistan deployed troops to "seal" the border because of the operation on the Afghan side. He spoke on condition of anonymity because he wasn't authorized to comment to media.

Some 2,500 Afghan and coalition forces backed by helicopter gunships last week began the offensive to flush out militants in Afghanistan's Kunar province bordering a tribal region of Pakistan.

Pakistan is an ally in the U.S.-led war on terrorism, but it has come under frequent criticism, particularly from Afghanistan, over the alleged movement of Taliban, Al Qaeda and other militants across the poorly marked border.