A bomb blast ripped through a bus terminal in a Pakistani region bordering Afghanistan on Friday, killed six people and scores injured, officials said, but there was no immediate claim of responsibility.

The device, apparently rigged to a motorcycle, exploded when a police patrol passed by the bus terminal in the northwestern city of Kohat, said police official Nauman Khan.

Police were investigating whether it was a remote controlled or a timed device, he added.

Five of the 13 wounded were in critical condition, said Ahsan Khan, an official at a district hospital.

Kohat is on the edge of country’s lawless tribal regions along the Afghan border.

Pakistan has launched several military operations there, disrupting the militants’ infrastructure and networks and substantially reducing violence, although the militants remain potent enough to launch attacks.