At least 20 Pakistani soldiers were killed on Sunday when an explosion hit an army convoy in the northwestern town Bannu, intelligence and military sources said.
More than 30 others were wounded in the attack on the convoy as it prepared to leave the town at the gateway to North Waziristan, a lawless, tribal region on the Afghan border where many militant groups are holed up.
"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 Pakistani Taliban claimed responsibility for the blast. The group had earlier vowed to step up attacks on security forces after electing a new hardline leader, Mullah Fazlullah, at the end of last year.
"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," Pakistani Taliban spokesman, Shahidullah Shahid, said in a telephone call from an undisclosed location.
A Pakistani 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."