Image caption Pakistan is one of only three countries where polio remains endemic

Gunmen have shot dead at least two policemen providing security to a team of polio workers in north-west Pakistan. A polio worker was also killed in a separate attack, local media have reported.

The two policemen were travelling from the town of Swabi to Topi by motorbike when they were attacked.

The polio worker was shot on the outskirts of Peshawar.

The attacks are the latest in a series targeting polio teams in the country.

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

Polio's last stand? 2012 cases Nigeria - 97

Pakistan - 47

Afghanistan - 26

Chad - 5* Source: IMB * Polio is considered "non-endemic" in Chad Polio killings a major setback

Islamist militants have been at the forefront of a decade-long campaign of violence against health workers, who they also accuse of being part of a Western plot to sterilise Muslims.

But earlier this week a prominent religious seminary in Pakistan - which wields great influence with Taliban groups - backed polio vaccinations, saying they were not un-Islamic.

Pakistan is one of only three countries where polio remains endemic, due in part to militant resistance to polio mass vaccination campaigns. Nigeria and Afghanistan are the others.

The current campaign against immunisation was started by the current Pakistani Taliban leader, Mullah Fazlullah, in 2005, and turned more violent after US forces killed former al-Qaeda chief Osama Bin Laden, in a secret raid in 2011.