Gunmen shot dead a policeman guarding health workers as Pakistan began another nationwide vaccination drive.

The policeman named Mohammad Sarfraz was shot dead in the Bajur region of the country's north west on the first day of a campaign to vaccinate 39 million children under 5.

Anwarul Haq, a local official, told Associated Press that suspects had been seized after the killing in the village of Badam.

There was no immediate claim of responsibility, but polio workers have in the past been targeted by militants claiming drops to immunise children against the crippling disease are in fact a Western conspiracy.

Scores of police and health workers were killed on polio duty earlier in the decade, but security has improved significantly since Pakistan's military began operations to clear the Taliban from the border regions.

Suspicion against the drops has also been cut by an education campaign and the use of locally-hired female health workers.