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.
Rahim said it was not immediately clear if the two workers killed Thursday were the actual target of the bombing.
No one has claimed responsibility for the attacks but suspicion fell on Islamic militants.
Some of the militants 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.
Pakistan is one of only three countries where the crippling disease is endemic. The virus usually infects children living in unsanitary conditions; it attacks the nerves and can kill or paralyze. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 the previous year, according to the United Nations.