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.
Rahim said it was not immediately clear if the two workers killed Thursday were the actual target of the bombing. Javed Husain, a doctor at a hospital in the town of Parachinar, said the slain men were working as contractors for the government-run anti-polio program in the area.
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.
Most of the new cases in Pakistan were in the northwest, where the presence of militants makes it difficult to reach children.
In December, gunmen killed nine polio workers in similar attacks across Pakistan, prompting authorities to suspend the vaccination campaign in the troubled areas. The U.N. also suspended its field operations in December as a result of the attacks, though it has since resumed some activities.
Also Thursday, a gunman opened fire on a vehicle carrying three local Sunni Muslim clerics in the southern city of Karachi, killing the three before fleeing the scene, said police officer Asim Qaimkhani.
No one claimed responsibility for the attack, which Qaimkhani said was likely sectarian.
Karachi is the capital of southern Sindh province and has witnessed scores of attacks against minority Shiites or majority Sunnis in recent years.
Associated Press writer Adil Jawad contributed to this report from Karachi, Pakistan.