Pakistan is one of only three countries where polio is endemic. Militants however accuse health workers of acting as spies for the U.S. and claim the vaccine makes children sterile. Taliban commanders in the troubled northwest tribal region have also said vaccinations can't go forward until the U.S. stops drone strikes in the country.
Insurgent opposition to the campaign grew last year after it was revealed that a Pakistani doctor ran a fake vaccination program to help the CIA track down al-Qaida chief Osama bin Laden, who was hiding in the town of Abbottabad in the country's northwest.
The Taliban have targeted previous anti-polio campaigns, but this has been a particularly deadly week. The government is in the middle of a three-day vaccination drive targeting high risk areas of the country as part of an effort to immunize millions of children under the age of five.
"Such attacks deprive Pakistan's most vulnerable populations especially children of basic life-saving health interventions," said a statement jointly released by the government and the U.N. "We call on the leaders of the affected communities and everyone concerned to do their utmost to protect health workers and create a secure environment so that we can meet the health needs of the children of Pakistan."
The women who were killed Tuesday three of whom were teenagers were all shot in the head at close range. Four of them were gunned down in the southern port city of Karachi, and the fifth in a village outside the northwest city of Peshawar. Two men who were working alongside the women were also critically wounded in Karachi.
The attacks in Karachi were well-coordinated and occurred within 15 minutes in three different areas of the city that are far apart, said police spokesman Imran Shoukat. In each case, the gunmen used 9 millimeter pistols. Two of the women were teenagers, aged 18 and 19, and the other two were in their 40s, he said.
Two of the women were killed while they were in a house giving children polio drops, said Shoukat. The other two were traveling between houses when they were attacked, he said.
On Monday another person working on the anti-polio campaign, a male volunteer, was gunned down in Karachi. Taliban militants also killed three soldiers in an ambush of an army convoy escorting a vaccination team in the northwest.
Officials in Karachi responded to the attacks by suspending the vaccination campaign in the city, said Sagheer Ahmed, the health minister for surrounding Sindh province. The campaign started on Monday and was supposed to run through Wednesday, he said.
Immunization was suspended in Karachi in July as well after a local volunteer was shot to death and two U.N. staff were wounded.
There were conflicting reports about whether the campaign was also temporarily suspended in Khyber Pakhtunkhwa province, where the fifth woman was killed Tuesday.
The statement released by the government and the U.N. said the drive was halted in both Sindh and Khyber Pakhtunkhwa.
But Janbaz Afridi, a senior health official in Khyber Pakhtunkhwa, said he had not received any suspension orders and planned to continue the campaign on Wednesday.
"These incidents are depressing and may cause difficulties in the anti-polio drive, but people should not lose heart," said Afridi. "The government is very serious, and we are determined to eliminate polio despite all odds and difficult conditions."
The shootings in Karachi all took place in areas mainly populated by ethnic Pashtuns, said Ahmed, the health minister. The Taliban are a Pashtun-dominated movement, and many militants are reported to be hiding in these communities in the city.
Rukhsana Bibi, whose 18-year-old daughter Madiha was killed in Karachi, seemed to blame the organizers of the vaccination campaign for her death.
"Why are you doing this by coming here?" said Bibi, standing next to her daughter's body at the hospital. "This is a prohibited area. Taliban are here."
Madiha was the only source of income for the family, which includes seven other children, said Bibi, whose husband is too sick to work.
The woman who was killed in the northwest was also a teenager and was shot by gunmen on a motorcycle as she was working with her sister in the village of Shinkai Hindkian, said Afridi, the local health official. She was rushed to the hospital after the attack but eventually died from her injuries, said Afridi.
Another official in Khyber Pakhtunkhwa, information minister Mian Iftikhar Hussain, said the woman was shot by her cousin because of a family dispute. He said she was working on the anti-polio campaign at the time, but he claimed the two things were unrelated.
Polio usually infects children living in unsanitary conditions, attacks the nerves and can kill or paralyze. Most of the new cases in Pakistan are in the northwest, where the presence of militants makes it difficult to reach children. A total of 56 polio cases were reported in Pakistan during 2012, said Ahmed, the Sindh health minister.
Despite the obstacles, the government has teamed up with U.N. agencies to give oral polio drops to 34 million children under the age of five. Clerics and tribal elders have been recruited to support polio vaccinations in an attempt to open up areas previously inaccessible to health workers
Also Tuesday, two men on a motorcycle hurled hand grenades at the main gate of an army recruiting center in the northwestern town of Risalpur, wounding 10 people, including civilians and security personnel, said senior police official Ghulam Mohammed.
Gunmen on a motorcycle also shot a member of an anti-Taliban militia in the northwest Swat Valley, said senior police official Gul Afzal Khan.
Pakistani Taliban spokesman Ahsanullah Ahsan claimed responsibility for both attacks by telephone to The Associated Press.
Abbot reported from Islamabad. Associated Press writers Jamal Khan and Rasool Dawar in Peshawar, Pakistan, Sherin Zada in Mingora, Pakistan, contributed to this report.