ISLAMABAD, Pakistan - Five women working for Pakistan's anti-polio campaign were slain Tuesday by gunmen on motorcycles, police said, on the second day of a three-day national immunization campaign.

Four of the women were killed in Karachi, the nation's largest city, and the fifth in Peshawar.

Sagheer Ahmed, the health minister for Sindh province, whose capital is Karachi, ordered a halt to the anti-polio drive in the port city after the shootings. Health officials said that vaccinations were also halted until further notice in Khyber Pakhtunkhwa province, where Peshawar is located.

Senior Karachi police officer Shahid Hayat told reporters that another polio worker was shot to death in the city Monday, although the circumstances of that male worker's death and its link to the vaccination program only became clear Tuesday. Hayat blamed the Karachi killings on "militants who issued a fatwa against polio vaccination in the past."

President Asif Ali Zardari and Prime Minister Raja Pervez Ashraf condemned the attacks. The president added that every community should cooperate fully with national polio eradication teams toward the goal of eliminating the disease.

Police said the women in Karachi were shot in the head at close range in apparently well-coordinated attacks using 9mm pistols. The killings occurred within 15 minutes of each other in three locations within the sprawling city.

As of late Tuesday, no one had taken responsibility for the attacks, said Sana Bari, Geneva-based spokeswoman for the World Health Organization, making it difficult to discern their motives.

In June, warlords and local leaders in North and South Waziristan banned the vaccination program, accusing health workers of spying for the United States. Leaflets distributed in the Wana subdivision of South Waziristan cited the case of Shakeel Afridi, a doctor who helped the CIA hunt down Osama bin Laden by conducting a fake vaccination program.