PESHAWAR, Pakistan - A suicide bomber pretending to need help with his car killed 26 people in northwest Pakistan yesterday, officials said, underscoring concerns that militant violence near the Afghan border could escalate now that Pakistan is shifting troops away from the region toward India.

The explosion at a polling station also wounded 15 people in Buner, a district bordering Swat, a valley where Pakistan’s army has waged a stop-start offensive against insurgents for more than a year, police official Beharmand Khan said. The building targeted was a school, where voters were casting ballots in a by-election for a National Assembly seat.