Two anti-polio workers were killed and five more injured in a string of attacks in northwest Pakistan today, taking the death toll to eight and prompting UN agencies to withdraw their workers from a vaccination campaign.

Teams involved in a nationwide anti-polio campaign were attacked at Peshawar, Charsadda and Nowshera in the restive Khyber-Pukhtunkhwa province, officials said.

A female anti-polio campaign supervisor and her driver were killed when gunmen fired at their vehicle in Shabqadar area of Charsadda this afternoon.

Five persons were injured in three separate attacks in Charsadda, Nowshera and Peshawar. One volunteer was seriously injured in an attack in Daudzai area of Peshawar.

Gunmen riding motorcycles attacked anti-polio teams in Charsadda and Nowshera. Gunmen warned female health workers in Charsadda that they would be killed if they continued with the anti-polio campaign, TV news channels reported.

Though authorities in Khyber-Pakhtunkhwa decided to press on with the anti-polio campaign, the World Health Organisation and UNICEF temporarily withdrew their workers from the drive.

A joint statement issued by the two organisations said: "In light of the prevailing security situation, WHO and UNICEF Pakistan are implementing additional security protocols to ensure the safety and security of their polio workers."

WHO and UNICEF said they remained committed to supporting the government and people of Pakistan in their efforts to rid the country of polio.

Yesterday, five female health workers were killed in attacks in Peshawar and Karachi. A male health worker was shot dead in Karachi on Monday.

WHO and UNICEF had yesterday said they would continue their anti-polio drive in Pakistan despite the killing of vaccinators but urged the government and society to do its "utmost to protect health workers".

Pakistan is one of only three countries where polio remains endemic, the others being Afghanistan and Nigeria.

No group has claimed responsibility for the attacks on anti-polio workers but the banned Pakistani Taliban have threatened vaccinators in the past.

Slain former premier Benazir Bhutto's daughter Aseefa Bhutto Zardari, the Pakistan government's Ambassador for Polio Eradication, strongly condemned the violence against health workers.

"We must not be deterred, better security precautions will be taken. We shall continue to fight in the name of those who have lost their lives for the cause," she said in a statement.

"Terrorists do this to scare us, they do this to try and stop us, we shall not be silenced and we will not stop, come what may," she said.