Militant group sees vaccination drive in Pakistan as plan by the United States to sterilise Muslims.
Health experts have condemned a "devastating" assault by the Pakistan Taleban in which five female polio vaccinators were shot dead in co-ordinated attacks across two cities.
The murders highlight the challenge for health workers in what is one of the world's last bastions of the crippling disease. Four of the women were shot dead by men on motorbikes yesterday in three separate areas of Karachi, within the space of 20 minutes. The fifth woman was killed in Peshawar. A sixth vaccination worker, a man, was killed on Tuesday.
The women were working on a three-day vaccination scheme backed by the World Health Organisation in areas where incidence of polio is the highest. The national drive, intended to give more than 5 million anti-polio drops, has been suspended in Karachi by the Government. Polio often thrives in places where sanitation is poor.
The ultimate aim of the project, which involves 90,000 health workers, is to provide drops to 35 million children. But the task of the Government and the aid organisations has become increasingly difficult since the Pakistan Taleban has become more outspoken in its opposition.
Sarah Crowe, Unicef's spokeswoman, said: "These attacks are a double tragedy. The work done to eradicate polio is pioneering. It has helped build up a foundation for stronger public health systems as health workers and polio vaccinators are often able to identify children who are missing out on routine immunisations.
"This comes at a time when Pakistan has made great strides against polio - last year 190 children contracted polio and this year it is 56. Every day the vaccination drive is on hold, more children lose out."
Sir Liam Donaldson, chairman of the independent monitoring board of the Global Polio Eradication Initiative, said: "The public health workers were doing heroic work to save children's lives in the final stage of the campaign to rid the world of a lethal and crippling disease.
"That they should lose their own lives in this appalling act of violence is devastating news for the global health community. In mourning their loss, we should honour their memories by showing their work will be carried on until every last child is safe from polio."
In many parts of Pakistan's north-west, the Taleban has banned such programmes, claiming they are a United States-backed plan to sterilise Muslims.
Antipathy to the vaccination drive has also increased since the CIA established a fake hepatitis drive in the city of Abbottabad to try to obtain information about Osama bin Laden.
Earlier this year, more than 200,000 children in North and South Waziristan missed out on being vaccinated after the Government failed to persuade militant leaders to allow staff to work there.
"There is a misconception that the vaccine could be harmful to the children; there is a misconception that the polio workers are spies; and there is a misconception that the polio drives are somehow linked to military operations in the north and in the tribal areas," Dr Guido Sabatinelli, head of the WHO's Pakistan's office, said from Islamabad.
Unlike India, which this year was declared polio-free, Pakistan is one of three nations where polio is a threat. The others are Nigeria and Afghanistan. The latest WHO figures suggest there have been 56 cases of polio confirmed in Pakistan this year with the results on about 300 other cases still outstanding.
In 2011, almost 200 children were paralysed by the disease, the most in 15 years.
Yesterday's shootings, which were condemned by Pakistan's Prime Minister, Raja Pervez Ashraf, took place in parts of Karachi dominated by Pashtuns, who have a large presence in the port city of 18 million people.
In addition to the women who were killed, two male workers were critically injured.