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Four polio vaccination workers have been shot dead and three injured in the south-west Pakistani city of Quetta.
Police said the three female health workers and their male driver were attacked by two gunmen on a motorbike. Anti-polio efforts in the province have been suspended, the authorities say.
The shootings are the latest in a series targeting Pakistani polio teams. No group has claimed responsibility.
Militants say polio teams are spies or that the vaccine causes infertility.
Pakistan is one of only three countries where the disease is still endemic, the others being Nigeria and Afghanistan.
Pakistan's National Co-ordinator for Polio Eradication, Ayesha Raza Farooq, told the BBC it was a "cowardly attack". She said the shootings happened as the workers walked a short distance to a rendezvous with their security detail.
"There was going to be security with this team before they went in and started their work. However, no one could have thought that this team would be intercepted," she said.
More than 60 Pakistani polio workers, or their guards and drivers, have been killed since 2012 - with most attacks blamed on Islamic extremists, the BBC's M Ilyas Khan reports from Islamabad.
Police officer Imran Qureshi told the BBC that two of the workers, both women, died on the spot, while two others died later of injuries sustained in the attack.
Pakistan has so far reported 260 new cases of polio this year, its highest number since 2000.
Poliomyelitis mainly affects children under five
Invades the nervous system and can cause total paralysis within hours
One in 200 infections leads to irreversible paralysis
Polio cases have decreased by more than 99% since 1988
Endemic in three countries - Afghanistan, Nigeria and Pakistan
There is no cure but the virus can be prevented by immunisation