A child receiving polio vaccine
Five female Pakistani polio vaccination workers have been shot dead in a string of coordinated attacks - four within 20 minutes across Karachi.
The fifth woman was killed in Peshawar. A male health worker was shot dead in Karachi on Monday.
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, reports the BBC.
"These were pre-planned and coordinated 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.
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,
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 18m.
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.
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.
Nearly 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 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 33m children for vaccination with some 88,000 health workers delivering vaccination drops.
2012 Polio Cases
Nigeria - 97
Pakistan - 47
Afghanistan - 26
Chad - 5