At least seven people were killed Wednesday in a bomb attack on a police van taking officers to guard polio vaccination teams in restive northwest Pakistan, officials said.
Militant attacks and threats of violence have badly affected efforts to stamp out the crippling disease in Pakistan, one of only three countries where it remains endemic.
Wednesday's blast in Sir Dheri, some 30 kilometres (20 miles) north of the main northwestern city of Peshawar came a day after gunmen killed three polio workers in Karachi.
Police said that up to five kilograms (11 pounds) of explosives were used in the bomb, which was detonated remotely.
"Seven people including six police personnel and a child were killed in a bomb blast in Sir Dheri," senior police official Shafi Ullah told AFP.
Another senior police official, Sayed Khan, told AFP the van was carrying personnel to a health centre, from where they were to accompany polio vaccination teams for security.
"We will continue providing security to polio workers as scheduled by the provincial government" he said.
An official of the district administration confirmed that the police team was on its way to provide security to polio workers when it came under attack.
Wednesday was the second day of an anti-polio drive in the area, senior local administration official Tahir Zafar told AFP.
Pakistan is one of only three countries in the world where polio is still endemic, along with Afghanistan and Nigeria.
Militant groups see vaccination campaigns as a cover for espionage, and there are also long-running rumours about polio drops causing infertility.
According to the World Health Organisation, Pakistan recorded 91 cases of polio last year, up from 58 in 2012.
On Friday the World Health Organization warned that Peshawar was the world's "largest reservoir" of polio.
Pakistan's struggles to defeat polio stand in stark contrast to its neighbour and great rival India, which last week celebrated three years since its last polio case.