Pakistan police officer guarding polio workers killed

A second police officer was injured in the attack

A police officer who was escorting a polio vaccination team in Pakistan has been shot dead, officials say.

Another officer was wounded in the attack in Mardan district, the latest in a series of attempts on the lives of polio workers and their escort teams.

No group has claimed responsibility for the killing, but the Pakistan Taliban has threatened health workers before.

Pakistan, Nigeria and Afghanistan are the only three countries in the world where polio is still endemic.

There were 35 cases of polio in Pakistan in 2012, according to the Polio Eradication Initiative.

The police officer was shot dead as he guarded a vaccination team which was inside a house giving polio drops. None of the health team were harmed.

BBC Pakistan correspondent Orla Guerin says that in the past the Pakistan Taliban has accused the vaccination teams of being American spies and of sterilising children with the vaccine.

In December, at least eight people engaged in polio vaccinations were shot dead in Karachi and the north-west, and in January and February two police officers were killed in similar attacks.

The UN said last month that some 240,000 children have missed vaccinations since July in parts of Pakistan's tribal region, the main sanctuary for Islamic militants, because of security concerns.