Gunmen have killed three more health workers, who were carrying out nationwide polio vaccinations across Pakistan, raising the death toll since the start of a United Nations campaign to nine.
In the latest attack, police say gunmen a female health worker and her driver were shot dead in the town of Charsadda, Peshawar in the country's northwest.
Another worker died after after being shot while giving out polio drops earlier on Wednesday.
Six other health workers, from other cities in Pakistan were killed in the past two days.
Senior police officer Shahid Hayat has blamed "militants who issued a fatwa against polio vaccination in the past" for the Karachi killings.
He says one polio vaccination team was attacked in the eastern Karachi neighbourhood of Gulshan-e-Buner.
"They were fired upon by unidentified gunmen who rode away on motorcycles," he said.
"Two women members suffered multiple gunshots and died on the spot,:
The health minister for the Sindh province, Sagheer Ahmed, of which Karachi is capital, says he had ordered a halt to the anti-polio drive in the city following the attacks
But a spokesman from UNICEF Australia, Tim O'Connor, told Radio Australia's Connect Asia the suspension of the vaccination program is temporary.
"It's an enormous concern to us because what we're trying to do is eradicate polio from not just Pakistan but the world," he says.
"It's a setback in the short term but UNICEF and the WHO - we're not going anywhere.
"We've got support from around the world including Australia and we really want to see this disease eradicated."
Pakistan is one of only three countries where polio remains endemic, along with Afghanistan and Nigeria, but efforts to tackle the highly infectious crippling disease have been hampered over the years by local suspicion.
The Taliban banned immunisations in the tribal region of Waziristan, condemning the campaign as a cover for espionage after the jailing of a doctor who helped the CIA find Osama bin Laden using a hepatitis vaccination programme.
Amnesty International's Mustafa Qadri told Connect Asia Pakistani authorities have to change this perception for the sake of public health.
"They have to much more counter the narrative by the Taliban that these polio vaccinations are actually an attempt to sterilise the population," he says.
A joint WHO-UNICEF statement says such attacks "deprive Pakistan's most vulnerable populations - especially children - of basic life-saving health interventions".
"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," it added.
UN spokesman Martin Nesirky says UN chief Ban Ki-moon condemned the "senseless and inexcusable attack on health workers" and was to meet Pakistan's UN ambassador Masood Khan on Tuesday over the issue.
US State Department spokeswoman Victoria Nuland also condemned the attacks, saying: "Any attack on health workers anywhere in the world is unacceptable."