A bomb rigged to a bicycle exploded in a northwest Pakistani city Monday, killing at least five people and wounding 20 others in the latest attack to rattle the volatile region, police said.

The explosion occurred on a major road in Dera Ismail Khan, and most of the victims were either walking by the parked bicyle or traveling in nearby vehicles, area police chief Saeed Ullah said.

Dera Ismail Khan lies in a region that neighbors the Waziristan tribal areas, where Taliban and Al Qaeda militants have strong sway and where the U.S. frequently strikes with missiles from across the border in Afghanistan.

But the rough and tumble city also has witnessed sectarian violence in the past.

Ullah refused to speculate on the motive behind the latest explosion, saying the investigation was continuing.

Local television channels showed footage of wounded victims being treated at a government hospital, as well as pieces of a bicycle, rickshaw and a damaged car at the scene of the blast.