Pakistani officials say a roadside bomb has killed two polio workers in a northwestern tribal region near the Afghan border.

Government administrator Yousuf Rahim says the men were going to a village to vaccinate children when the bomb hit their motorcycle in Kurram tribal region on Thursday.

Although it's unclear whether the workers were the actual target, it was the second attack this week against polio workers. On Tuesday, gunmen riding on a motorcycle killed a policeman protecting a polio vaccination team.

Pakistan is one of three countries where the crippling disease is endemic.

Suspicion for the attacks falls on Islamic militants, who oppose the vaccination campaign, accuse health workers of acting as spies for the U.S. and claim the polio vaccine is intended to make Muslim children sterile.