A policeman providing security for a polio vaccination team in Pakistan has been killed by gunmen near the north-western town of Swabi, officials say.
The killing is the latest in a spate of deadly attacks against vaccination workers in the country.
In December at least eight people engaged in polio vaccinations were shot dead in Karachi and the north-west.
No group has said it carried out Tuesday's attack, but the Taliban have threatened anti-polio efforts.
The militants have accused health workers of working as US spies and say the vaccine makes children sterile.
Along with Afghanistan and Nigeria, Pakistan is one of only three countries where polio is still endemic.
The latest attack took place on the southern outskirts of Swabi town, police say.
Tuesday was the second day of an immunisation drive in the area and a two-women team were administering polio drops to children.
"The team, after finishing the campaign in Kala [village of Swabi district] was heading towards a nearby village when three men armed with Kalashnikovs appeared from sugarcane fields and opened fire," Swabi police chief Abdul Rashid Khan told the AFP news agency.
He said that the two team members were unhurt and the gunmen escaped.
"It seems the target was the policeman," Mr Khan said.
Health officials say a total of 538 immunisation teams were deployed in Swabi district on Monday, each accompanied by a policeman for protection.
The BBC's M Ilyas Khan in Islamabad says Swabi has had its share of militant attacks in the past, mostly targeting non-governmental organisations involved in health and education projects.
On 1 January seven charity workers, six of them women, were shot dead in the Swabi area. Correspondents say it is not clear if they were targeted because their charity offered vaccinations or education for girls.
All NGO operations in Swabi district have now been suspended, officials say.