Five workers with the vaccination campaign against polio in Pakistan were shot to death on Tuesday in three separate attacks in the cities of Karachi and Peshawar, authorities told Efe.

Four of the murdered people worked in Karachi, where shortly before midday they were gunned down in two attacks staged within a span of 30 minutes by unknown gunmen riding on a motorcycle, according to police sources.

Shortly thereafter, a woman working with the vaccination campaign in Peshawar was shot to death under similar circumstances, police said.

An official with the Pakistani government entity that coordinates the fight against polio told Efe that the vaccination campaigns have been temporarily suspended in both cities.

The fight against polio in Pakistan - one of only three countries where the disease is still endemic - has been surrounded by controversy since the Taliban insurgency and some groups in the country's tribal areas expressed their opposition to it.

One of the main causes of that opposition was the revelation that the CIA established a phony vaccination drive in the course of its ultimately successful efforts to track down Osama bin Laden.

Last summer, Taliban members in the tribal areas in northwestern Pakistan threatened to attack the people conducting the polio vaccination campaign.

Incidents linked to the vaccination campaign have been registered at various points around the country and in July a member of the campaign was shot to death in Karachi. EFE