Health campaigners are struggling to vaccinate vulnerable children in the face of opponents who claim immunisations are part of a Western plot against Muslims and that intelligence agencies are using health workers as spies.

The latest killings were in the lawless tribal belt that borders Afghanistan.

"The two-men team was visiting Mali Khel village in upper Kurram tribal region when an improvised explosive device planted along the roadside went off, killing them on the spot," Jawad Ali, in charge of the campaign in Kurram, told AFP.

Pakistan , Afghanistan and Nigeria are the last three countries where polio remains endemic.

Insecurity along the Afghan border means vaccination teams struggle to reach children at risk. Taliban commanders have also banned teams from visiting their areas, demanding that drone strikes cease before permission is granted.

The situation has deteriorated since the CIA ran a hepatitis campaign in Abbottabad, as part of a ruse to determine whether Osama bin Laden was living there in 2011.

Since then, a number of influential hardline religious leaders have withdrawn their support for vaccination saying they could not be sure that immunisation teams were not gathering intelligence.