PESHAWAR, Pakistan - The United Nations suspended its polio vaccination drive in Pakistan on Wednesday after eight people involved in the effort were shot dead in the past two days, a U.N. official said.

The suspension was a grave blow to the drive to bring an end to the scourge of polio in Pakistan, one of only three countries where the crippling disease still survives.

On Wednesday, gunmen shot at a woman working on the campaign in northwest Pakistan, killing her and her driver, one of five attacks during the day on polio workers.

A male polio immunization worker was critically wounded in one of the shootings.

This week six other people have been killed who were working on the immunization program, which has been jointly conducted with the Pakistani government. No one has claimed responsibility, but some Islamic extremists claim that the program is a cover for espionage.

U.N. Secretary-General Ban Ki-moon condemned the killing as "cruel, senseless and inexcusable."

He said the eight workers were among thousands across Pakistan "working selflessly to achieve the historic goal of polio eradication."

Sarah Crowe, spokeswoman for UNICEF, said the vaccination program has been suspended everywhere in Pakistan until an investigation by the Pakistani government is completed.

"This is undoubtedly a tragic setback, but the campaign to eradicate polio will and must continue," she said.

Some provincial governments in Pakistan continued to immunize children, independent of the U.N. drive.

Prevention efforts have reduced the number of cases in Pakistan by about 70 percent this year compared with 2011, but the recent violence threatens to reverse that progress.

Suspicion for the attacks has fallen on the Pakistani Taliban because of its virulent opposition to the polio campaign, but the group's spokesman, Ahsanullah Ahsan, denied responsibility in a telephone call to the Associated Press.