Pakistan's border with Afghanistan was closed Thursday and security forces were on alert as Afghans voted in national elections under threat of Taliban attacks, officials and residents said. An insurgency by the Islamist extremists plagues both Muslim states, with militants slipping over the porous border and also staging attacks on convoys supplying NATO troops as they travel from Pakistan to war-torn Afghanistan. "The Pak-Afghan border has been completely sealed from Tuesday and will be closed till Friday... security is on high alert on both sides of the border," said Rehan Gul Khattak, assistant administrative chief for Khyber district. "The containers carrying goods for NATO forces have also been stopped and there are long queues of supply vehicles on the border." Khattak was referring to the official border posts, but much of the traffic along the 2,500-kilometre (1,500-mile) frontier is through informal crossings or deep in the remote mountains where there is no government presence.