At least 20 Pakistani soldiers were killed on Sunday when an explosion hit an army convoy in the northwestern region of Bannu, intelligence and military sources said.
At least two dozen others were wounded in the attack in a town near North Waziristan, a lawless Pakistani region on the Afghan border where many militant groups are holed up in remote mountain camps.
"It's not clear if it was a suicide attack or an improvised explosive device," said one military official, adding that the soldiers had been travelling in a civilian vehicle hired to carry them to Razmak in North Waziristan.
The assault follows a threat by the Pakistani Taliban to step up attacks on security forces after the group elected a new leader, Mullah Fazlullah, at the end of last year.
Another police officer said the blast took place as a convoy of army troops and paramilitary Frontier Corps was preparing to leave Bannu, adding that security forces had cordoned off the area to investigate.
"We fear that dozens were killed and injured," said an intelligence source. "It is not yet clear what caused the blast ... A large number of soldiers were there."
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