PESHAWAR, Pakistan — At least two police officers were killed and a dozen people wounded on Monday when a bomb went off near a health care facility where polio vaccines were being dispensed outside this northwestern Pakistani city, officials said.

The bomb exploded just as a police escort for a vaccination team reached the site in the village of Sulmankhel in Peshawar’s southern suburbs, said Najibur Rehman, a Peshawar police superintendent. “Just when our mobile squad reached the health dispensary, a bomb planted in the nearby field went off,” Mr. Rehman said by telephone from the scene.

Officers were trying to defuse a second bomb planted nearby, said Shafqat Malik, an officer with the bomb disposal squad, whose team had also been accompanying the vaccination workers.

Polio vaccination workers in Pakistan have been the targets of a militant-led campaign of intimidation and violence. Some militants have accused polio workers of using vaccination campaigns as a cover to spy on behalf of the United States.