Gunmen in Pakistan killed five women today, who were working for a UN sponsored polio vaccination campaign.
The women were shot in a series of co-ordinated attacks in two different cities.
Four were killed in Karachi, in three different parts of the city. The fifth woman was shot and wounded in the city of Peshawar and later died.
"These were pre-planned and co-ordinated attacks in various localities which took place within a span of 20 minutes," Imran Javed, a police spokesperson told the BBC.
Three of the women killed today were teenagers, aged 17, 18 and 19. They were shot in the head at close range.
Two male workers were critically wounded. Yesterday, a young man working for the World Health Organization's campaign was shot and killed.
No group has claimed responsibility for the attacks. But the Taliban has issued threats against the polio drive, claiming the vaccine is dangerous and the health workers are spies for the U.S.
Last year, a Pakistani doctor set up a phony hepatitis vaccination campaign to help the CIA find Osama Bin Laden.
The Pakistani government was in the middle of a three-day campaign to give oral polio drops to more than 30 million people, particularly children under the age of five.
Pakistan is a key country in the global fight against polio - which attacks the nervous system, most commonly in children.
It can kill or cause permanent paralysis within hours.
Pakistan is one of only three countries where the disease is still common, along with Afghanistan and Nigeria.
Nearly 200 children were paralyzed in Pakistan last year - the most in 15 years.
For now, the government has suspended the polio drive in Karachi, Pakistan's largest city with 18 million people. It was supposed to run until tomorrow.
The campaign was suspended in Karachi in July as well after a local volunteer was shot and killed, and two U.N. staff were wounded.
"These incidents are depressing and may cause difficulties in the anti-polio drive, but people should not lose heart," said Janbaz Afridi, a senior health official.
"The government is very serious, and we are determined to eliminate polio despite all odds and difficult conditions."
Earlier this year, the World Health Organization said polio was at a tipping point, after large outbreaks in Africa, Tajikistan and China's first cases in more than ten years.