Five female Pakistani polio vaccination workers have been fatally shot in a string of co-ordinated attacks - four within 20 minutes across Karachi.
The fifth woman was shot and wounded in the city of Peshawar in the north-west and later died of her injuries.
A UN-backed programme to eradicate polio - which is endemic in Pakistan - has been suspended in Karachi.
No group has said it carried out the shootings, but the Taliban have issued threats against the polio drive.
"These were pre-planned and co-ordinated attacks in various localities which took place within a span of 20 minutes," Imran Javed, a police spokesman told the BBC of Tuesday's attacks in Karachi.
Earlier reports said a male health worker had been shot dead in Karachi on Monday, but officials now say his death was not related to the polio vaccination drive.
Prime Minister Raja Pervez Ashraf has condemned the attacks and praised the work of the polio vaccination teams, calling on regional authorities to guarantee their safety, Pakistan's APP news agency reported.
Pakistani health officials said the latest three-day nationwide anti-polio drive - during which an estimated 5.2 million polio drops were to be administered - had been suspended in Karachi, Pakistan's largest city with a population of 18 million.
There has been opposition to such immunisation drives in parts of Pakistan, particularly after a fake CIA hepatitis vaccination campaign helped to locate Osama Bin Laden in 2011.
Militants have kidnapped and killed foreign NGO workers in the past in an attempt to halt the immunisation drives, which they say are part of efforts to spy on them.
However, the Pakistani government "would continue to mount its effort on polio eradication," Mr Ashraf's special adviser Shahnaz Wazir Ali told the BBC.
Mrs Ali said protection would be provided to workers, and campaigns would be staggered if necessary.
"Clearly, we are now so close to eradicating the polio virus,... acts of this type, which are intended to dissuade us, will not deter us," she said.
Along with Afghanistan and Nigeria, Pakistan is one of only three countries where polio is still endemic.
Pakistan is considered the key battleground in the global fight against the disease, which attacks the nervous system and can cause permanent paralysis within hours of infection.
Almost 200 children were paralysed in the country in 2011 - the worst figures in 15 years.
Earlier this year, the Global Polio Eradication Initiative warned that tackling the disease had entered "emergency mode" after "explosive" outbreaks in countries previously free of polio.
The World Health Organization (WHO) said polio was at a tipping point, with experts fearing it could "come back with a vengeance" after large outbreaks in Africa and Tajikistan and China's first recorded cases for more than a decade.
Declaring polio a national emergency, the Pakistani government is targeting 33 million children for vaccination with some 88,000 health workers delivering vaccination drops.
Dr Bruce Aylward of the WHO told the BBC that vaccination programmes had been suspended in other countries before but that "when you're dealing with something as basic as the health of children, usually there can be common ground found".
Dr Aylward said he hoped for a "dialogue with community leaders who have positions of power to ensure root causes of this are being addressed and the perpetrators are brought to justice".