Around a dozen other police officers were injured in the attack, in which two vehicles are reported to have been hit by bombs planted along the side of the road they were travelling along in the Jamrud tribal region in Khyber Pakhtunkhwa province.

The Associated Press cited a local official who said that no-one was killed but several police officers were wounded when the first bomb struck a vehicle escorting the vaccination team.

The fatalities came a few minutes later, when the second bomb hit a convoy of tribal police officers which had been sent to pick up the victims of the first attack. The official said that 11 were killed in the bombing, and gunmen had opened fire on the policemen who emerged unhurt.

A separate official said a manhunt had been launched in an effort to find and arrest the attackers.

Frequent targets

There was no immediate claim of responsibility for the attack, but polio-vaccination teams have frequently been targeted by Islamist militants in Pakistan, who claim that such campaigns are a cover for spying activities, while at the same time making boys sterile.

Pakistan is one of the few country where polio remains prevalent, and according to the World Health Organization, in 2013 it was the only country in the world in which the disease was on the rise. Most polio cases are recorded in Pakistan's northwestern tribal region, which is difficult for vaccinating teams to reach.

The country's neighbor and chief rival, India, meanwhile, recently celebrated being declared free of the disease, after three years without a new diagnosed case.

pfd/tj AFP, dpa, AP