Gunmen in Pakistan opened fire on health workers carrying out polio vaccinations in three separate attacks on Wednesday.
Pakistan is one of only three countries in the world where polio is still endemic, but efforts to stamp out the highly infectious disease have been hampered by resistance from the Taliban, who have banned vaccination teams from some areas.
Six people working to immunize children against the crippling disease have been shot dead in Pakistan since a nationwide UN-backed vaccination campaign began on Monday.
Two workers were giving out polio drops in a suburb of Peshawar, in the northwest, when gunmen on a motorbike rode up and sprayed them with bullets, said Janbaz Afridi, doctor in charge of the immunization campaign in Khyber Pakhtunkhwa province.
One of the workers was in critical condition in hospital after the attack, he said.
Senior police official Asif Iqbal confirmed the attack and said police were investigating.
Two other polio teams were targeted in similar attacks in the towns of Nowshera and Charsadda, police and health officials said, but the workers escaped unharmed. One passerby was slightly injured in Nowshera.
Violence has blighted every day of the polio campaign so far: one health worker was shot dead in Karachi on Monday and four more were killed in the city with another gunned down in Peshawar on Tuesday.
After Tuesday's attacks, the government of Sindh province, of which Karachi is capital, halted the immunization drive, and Afridi said the Khyber Pakhtunkhwa government would have no choice but to follow suit.
"We are holding an emergency meeting with our donors and we have no option but to postpone the vaccination campaign for the time being. We will formally announce it after the meeting," he said.
Rumors about polio drops being a plot to sterilize Muslims have long dogged efforts to tackle the disease in Pakistan, but suspicion of vaccination programs intensified after the jailing of a doctor who helped the CIA find Osama bin Laden in 2011 using a hepatitis vaccination program.
There has been no claim of responsibility for the polio worker attacks, but in June the Taliban banned immunizations in the tribal region of Waziristan, condemning the polio campaign as a cover for espionage.
In Waziristan, a hub for Islamist militants, the ban – also enforced as a protest against US drone strikes – has put the health of 240,000 children at risk, officials say.