Seven policemen have been killed in Pakistan by unidentified gunmen while guarding medical workers administering polio vaccinations. 

The attacks took place within 600 metres of each other near a market in Karachi. Teams in Pakistan working to immunize children against polio are often targeted by Taliban and other militant groups. 

The militants say the campaign is a cover for Western spies, or accuse workers of distributing drugs designed to sterilise children.