The United Nations suspended its immunization drive in Pakistan on Wednesday following a spate of deadly attacks on polio vaccination workers in the past two days.

Two health workers – a vaccination supervisor and her driver – were gunned down in the Peshawar region, bringing the number of health workers killed in this week’s anti-polio drive to eight. Most attacks on health workers have taken place in areas near militant strongholds.

No group has claimed responsibility, but some Islamic extremists accused the immunization drive is a cover for espionage and say the vaccine makes children sterile.

UN Secretary-General Ban Ki-moon condemned the killing as “cruel, senseless and inexcusable” and described the eight workers as among thousands across Pakistan “working selflessly to achieve the historic goal of polio eradication”.

Vaccination programs across the country have been suspended until the government concludes an investigation. Pakistan is one of just three countries where the crippling disease is still widespread.

“This is undoubtedly a tragic setback, but the campaign to eradicate polio will and must continue,” UNICEF spokeswoman Sarah Crowe told AP.

According to Reuters, despite the killings, some provincial governments independent of the UN continued to immunize children.

On Tuesday, four female health workers were shot dead in Karachi and a fifth woman died after being shot in Peshawar. A male polio vaccination worker was also killed in Karachi a day earlier, BBC reports.