PARACHINAR, Pakistan (AP) — A roadside bomb killed two Pakistani polio workers on their way to vaccinate children in a northwestern tribal region near the Afghan border on Thursday, an official said.

The two men were on their way to Malikhel village as part of the U.N.-backed anti-polio campaign when the bomb hit their motorcycle, said government administrator Yousuf Rahim.

The attack – the third this week against polio workers in Pakistan – took place in the Kurram region, a known militant stronghold.

On Tuesday, gunmen riding on a motorcycle shot and killed a policeman protecting a polio team in Gullu Dheri village of Swabi district in Khyber Pakhtunkhwa province. The polio workers escaped unharmed in that attack.

In a separate incident in the northwest on Tuesday, a man wounded a polio worker with an axe.