Khyber Pakhtunkhwa: At least five police officers have been killed and nearly two dozen others wounded after an explosive device targeted their vehicle during a polio vaccination drive in north-western Pakistan.

The blast took place early morning on Monday in Bajaur, a tribal district in Khyber Pakhtunkhwa province neighbouring Afghanistan, when Pakistan began its latest round of the vaccination campaign.

Five of those wounded were in critical condition and have been shifted to the provincial capital, Peshawar, about 133km (82 miles) south of Bajaur.

“The blast took place early in the morning when the police team went out for the polio vaccination drive. We have shifted most of the injured to local hospital in Bajaur while critical patients were sent to Peshawar,” Bilal Faizi, spokesperson for the provincial rescue services, said.

There was no immediate claim of responsibility for the attack.

In recent years, the Pakistan Taliban, also known by the acronym TTP, has killed dozens of polio vaccination workers and security officials in Pakistan.

Resistance to the polio immunisation drive grew in Pakistan after the CIA, the intelligence agency of the United States, organised a fake vaccination drive to track Osama bin Laden, who was killed in the Pakistani garrison town of Abbottabad in 2011.

Religious leaders in the Pakistan-Afghanistan border region also spread misinformation that the vaccine contained traces of pork and alcohol, which are forbidden in Islam.

Pakistan and Afghanistan are the only two countries yet to be declared free of the wild poliovirus type 1 (WPV-1) disease.

For a country to be recognised “free of polio”, it should show an absence of WPV-1 transmission for at least three consecutive years, according to the Global Polio Eradication Initiative.