Gunmen on motorcycles have opened fire on police escorting a team of polio workers during a door-to-door vaccination campaign in north-western Pakistan, killing an officer and a polio worker, officials said.

No-one immediately claimed responsibility for the attack in Bajur, a district in the Khyber Pakhtunkhwa province and a former stronghold of the Pakistani Taliban, according to local police chief Abdul Aziz.

Pakistan’s interior minister Mohsin Naqvi issued a statement condemning the attack.

A nationwide polio campaign was launched on Monday amid a rise in militant attacks. The potentially fatal, paralysing disease mostly strikes children under the age of five and typically spreads through contaminated water.

That same day, a roadside bomb hit a vehicle carrying officers assigned to protect health workers conducting polio immunisation in the north-western South Waziristan district, in the same province, injuring six officers and three civilians.

The militant so-called Islamic State group later claimed responsibility for Monday’s attack.

Anti-polio campaigns in Pakistan are regularly marred by violence.

Militants target vaccination teams and police assigned to protect them, falsely claiming that the campaigns are a Western conspiracy to sterilise children.

Since January, Pakistan has reported 17 new cases of polio, jeopardizing decades of efforts to eliminate polio in the country.

Pakistan and Afghanistan are the only countries in which the spread of polio has never been stopped.