In Saturday's incident, a team of polio workers was being driven to Landi Kotal tehsil in Khyber Agency for a camp when gunmen fired at them. The driver of the vehicle was killed, a media report said.

The missing team was headed for a remote area of Balochistan province.

"Two polio workers, escorted by two constables of the Balochistan Levies, left the district headquarters at 11 p.m. On Friday to vaccinate children in the Murgha Gibzai area of Zohab district but they have not reached their destination," Zhob Deputy Commissioner Nazir Ahmed Khetran said.

"It is a remote area, there is no proper communication system, which is why we are facing difficulty in tracing them," he said.

Vaccinators often go to the remote areas and return the next day, an officer of the Levies, a paramilitary force, said.

The polio team was supposed to conduct a vaccination campaign in Murgha Gibzai, Toda Ghibzai and Barkwal union councils. Zhob is one of the "high-risk" areas when it comes to polio vaccination.

Pakistan is one of only three countries in the world where polio remains endemic, along with Afghanistan and Nigeria. Efforts to eradicate the disease have been seriously hampered by the targeting of vaccination teams in recent years.

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