At least 20 soldiers have been killed in north - western Pakistan in a bomb blast that struck an army convoy, sources in the security forces say.At least 24 others were injured in the explosion near the town of Bannu, with fears the death toll will rise. The source of the explosion is still being investigated. Pakistan's Taliban said they had carried out the attack as part of a "fight against a secular system" and promised "many more such attacks". Soldiers and paramilitary Frontier Corps were preparing to leave Bannu, Khyber Pakhtunkhwa province, for Razmak in North Waziristan when their convoy was rocked by the blast. "The explosion took place in one of the vehicles of the convoy," an unnamed senior security official told AFP. "We are trying to ascertain the exact nature of the explosion, whether it was a planted device or a suicide attack." A military source told Reuters news agency the soldiers had been travelling in a hired civilian vehicle. Speaking to Reuters by telephone from an undisclosed location, Taliban spokesman Shahidullah Shahid said: "With the help of God we claim responsibility for this. "The army is our enemy. We will carry out many more attacks like this again." Last week, a senior police officer known for campaigning against the militants was killed in a bomb blast in Karachi.