DERA ISMAIL KHAN, Pakistan — A bomb rigged to a bicycle exploded in a northwest Pakistani city on Monday, killing at least five people and wounding 20 in the latest attack to rattle the volatile region.

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

Taliban and al-Qaida militants are suspected in scores of attacks in Pakistan's northwest regions bordering Afghanistan. Dera Ismail Khan lies near the Waziristan tribal areas, which are insurgent strongholds and favorite targets for U.S. missile strikes.

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.

In Pakistan's southwest, gunmen shot dead the leader of a small Shiite political party in the main city of Quetta, triggering violent protests. Several hundred people torched vehicles and a bank, said Mohammed Khan, an area police official.

Photos from the scene showed security forces clashing with protesters. Khan said efforts were under way to restore order.

Ali Haider, a purported spokesman for the outlawed Sunni Muslim militant group Lashkar-e-Jhangvi, said the group was behind the killing in a phone call to the local press club.

Elsewhere in the northwest, a man whom militants accused of spying for America was found shot dead Monday in Datta Khel village in North Waziristan, two intelligence officials said. The body was missing the right hand, which had been chopped off.

A note pinned to the body claimed the man spied for the U.S. and warned others to "learn a lesson from the fate of this man," said the officials, who spoke on condition of anonymity because they were not authorized to speak to media.

Militants in the lawless region have killed at least 20 alleged spies since mid-December.