Police in northwestern Pakistan say an explosion has killed one police officer and wounded another officer who were guarding polio vaccination workers.
The blast happened Sunday in the Budhni area, on the outskirts of Peshawar, the gateway to the country's restive tribal regions where the military has been battling Taliban and al-Qaida-linked militants.
Militants frequently attack polio workers in Peshawar and adjoining tribal districts accusing them or being Western spies or part of a plot to sterilize Muslims.
The World Health Organization has declared Peshawar as the world's "largest reservoir" of polio.
Tests found that 83 of the 91 polio cases in Pakistan last year were genetically linked to strains in Peshawar. Twelve of 13 polio cases in Afghanistan were also linked to Peshawar.
Pakistan is one of three countries where polio remains endemic. However, it is the only country that experienced a rise in polio victims last year.
Polio is also endemic in Afghanistan and Nigeria.
The Global Eradication Initiative says until poliovirus transmission is interrupted in these countries, all countries remain at risk of importation of polio.