At least nine polio immunization workers were shot to death in northern Nigeria on Friday by gunmen who attacked two clinics, officials said.

The killings, with eerie echoes of attacks that killed nine female polio workers in Pakistan in December, represented another serious setback for the global effort to eradicate polio.

Most of the victims were women and were shot in the back of the head, local reports said.

A four-day vaccination drive had just ended in Kano State, where the killings took place, and the vaccinators were in a “mop-up” phase, looking for children who had been missed, said Sarah Crowe, a spokeswoman for the United Nations Children’s Fund, one of the agencies running the eradication campaign.

Dr. Mohammad Ali Pate, Nigeria’s minister of state for health, said in a telephone interview that it was not entirely clear whether the gunmen were specifically targeting polio workers or just attacking the health centers where vaccinators happened to be gathering early in the morning. “Health workers are soft targets,” he said.