ISLAMABAD (Deutsche Presse-Agentur) -- Authorities suspended a polio immunization campaign Tuesday after six Pakistani health workers were shot dead, officials said.

Efforts to eradicate the crippling disease have met with resistance in the past due to rumours that the vaccine causes infertility.

Four female health workers administering drops of the oral vaccine were killed in attacks on three locations in the southern port city of Karachi, capital of Sindh province, police spokesman Ghaffar Shah told dpa over the phone.

Two male colleagues were also injured in the shootings, which took place in Pahstun-dominated neighborhoods.

Also Tuesday, one female vaccinator was killed in the north-western city of Peshawar, police said.

"The woman suffered fatal injuries when an unknown gunman fired at her as she was going for her duty this morning," Peshawar police spokesman Jalaluddin told dpa.

Late Monday in Karachi, gunmen shot dead a male volunteer in the immunization campaign as he was returning home.

The provincial health minister ordered the suspension of an ongoing three-day anti-polio campaign throughout Sindh following the killings, a senior government official Saleem Khan said.

"A decision to restore the immunization drive will be taken after assessing the (security) situation," he said.

A similar campaign was also suspended in July following the killing of a Pakistani staffer and a gun attack that injured a UN official from Ghana and his Pakistani driver.

Sharjeel Memon, information minister in Sindh, told the state-run Pakistan Television news channel that security forces would be deployed with immunization teams visiting vulnerable areas in future.

Pakistani Prime Minister Raja Pervez Ashraf condemned the attacks on vaccinators, directing the federal interior ministry to coordinate with the polio eradication officials to ensure "foolproof security" for the health teams.

"We cannot and will not allow polio to wreak havoc the lives of our children," Ashraf told a meeting in Islamabad.

A government statement said authorities also discussed plans to immunize 34 million children against polio next year by involving over 25,000 workers, including female staff who have worked for the anti-polio campaign.

Pakistan is one of only three remaining countries where polio is still prevalent.

The World Health Organization's statistics show that 58 polio cases have been reported in Pakistan so far this year. The total number of cases last year was 198.

The vaccination campaign has been hindered by the infertility rumours, and also after a Pakistani doctor used a polio immunization drive as cover to help US intelligence track down al-Qaeda leader Osama bin Laden in the northern town of Abbottabad last year.

No one took responsibility for the recent attacks. Taliban militants this year banned the UN-backed anti-polio campaign from a north-western tribal region.

Copyright 2012 dpa Deutsche Presse-Agentur GmbH