A bomb struck a military convoy in Pakistan's restive northwest on Sunday, killing 20 soldiers and wounding more than 30, officials said. The attack occurred in the city of Bannu, close to the troubled North Waziristan tribal region which is a known stronghold of Taliban and al-Qaeda-linked militants. "We are trying to ascertain the exact nature of the explosion, whether it was a planted device or a suicide attack," a senior security official told AFP. "The explosion took place in one of the vehicles of the convoy killing 20 soldiers and wounding more than 30," he said. The official said "the convoy was about to leave for Razmak town in North Waziristan when the tragedy struck", adding the blast occurred in one of the civilian vehicles hired for the movement of troops. A Frontier Corps official also confirmed the attack and casualties. Bannu has been the site of several attacks targeting security forces in the past. Pakistani troops have been fighting for years against homegrown insurgents in the tribal belt, which Washington considers the main hub of Taliban and al-Qaeda militants plotting attacks on the West and in Afghanistan.