Media playback is unsupported on your device Media caption The BBC's Shahzeb Jillani: "There have been previous attacks on polio teams, a lot of people were reluctant to carry out this campaign"

A bomb attack on a polio vaccination team in north-west Pakistan has killed at least 11 people, officials say

A roadside bomb went off as the police-guarded convoy drove through a village in Khyber Pakhtunkhwa province near the Afghan border.

The attack is the latest in a series targeting polio teams in the country.

No group has claimed responsibility, but the Taliban oppose the polio schemes, which they see as a cover for international espionage.

Initial reports said the convoy was struck by two separate bombs on Saturday.

The blasts were reportedly followed by a fierce gun battle between security forces and the militants.

Officials confirmed that people wounded in the attack were taken to a nearby hospital. Some of those injured are said to be in a critical condition.

Pakistan has witnessed a campaign of violence against health workers, who militants also accuse of being part of a Western plot to sterilise Muslims.

More than 40 people linked to the vaccination programme have been killed in Pakistan since December 2012.

Last month, unidentified gunmen shot dead three polio workers in the southern city of Karachi, a day after authorities began a new nationwide vaccination drive.

Pakistan is one of only three countries where polio remains endemic, along with Nigeria and Afghanistan.

According to the World Health Organization, Pakistan recorded 91 cases of polio last year, up from 58 in 2012.