Two female polio vaccinators were shot dead in northwestern Pakistan in the latest attack on the global campaign to eradicate the crippling virus.

Unidentified gunmen on motorcycles opened fire at the polio team in the town of Swabi. One woman, identified only as Shakeela, died immediately and another, Ghuncha, died later of her wounds in a Peshawar hospital.

There was no immediate claim of responsibility for the attack.

The shootings came as Pakistan is trying to redouble its campaign to vaccinate children after a sudden jump in cases and international fears Islamabad is losing grip on a virus that appeared close to extinction.

Pakistan had hoped it was within reach of wiping the virus out in 2018, when only 12 cases were reported across the nation of more than 210 people. Pakistan and neighbouring Afghanistan are currently the only two countries in the world with new cases of the wild virus.

Yet cases last year leapt to 140 as international monitors raised concerns the programme had become riven by political infighting and officials were failing to overcome stubborn public distrust of the campaign.

Officials are still confident the virus can be wiped out, but admit the "last mile" is proving longer and more difficult than expected.

Vaccination teams face widespread conspiracy theories that the vaccination campaign is a Western plot to sterilise Muslims, or that the drops can poison children. Militants also stepped up attacks on polio workers after it was disclosed the CIA used a fake hepatitis vaccination campaign to take DNA samples in the hunt for Osama bin Laden in Pakistan. Moreover, poverty stricken villages resent the government insistence on taking polio drops, when they say the state fails to address basic needs like water and electricity.

Local women are recruited as health workers by the door-to-door campaign because they have the trust of their communities, but they have found themselves frequently under attack.

Pakistan has seen a further six cases in 2020 already. Sewage samples taken from cities across the country show the virus is still widespread and officials fear cases may be high again this year.