ISLAMABAD - Armed men attacked policemen providing security for polio vaccination workers in Pakistan's northwest Khyber Pakhtunkhwa province, killing a police officer while injuring another, police officials said on Tuesday.

The incident happened when a group of terrorists opened fire on the polio team in the Dabori area of Orakzai district of the province, police sources in the area told Xinhua.

Police said that two terrorists were also killed in the ensuing exchange of firing between security forces and the attackers, adding that the law enforcers have launched an operation in the surrounding areas to arrest those who fled.

No group or individual has claimed the attack yet.

Earlier on Monday, Pakistan launched a nationwide vaccination campaign to vaccinate more than 45 million children in various districts, according to a statement from the country's National Institute of Health.