Unidentified assailants shot and killed on Wednesday two police officers assigned to protect polio vaccination workers in Mardan district of Khyber Pakhtunkhwa, police said.

The victims, identified as police constables Shakir and Syed Raza, were on security duty for the polio team and were on their way back to the police station on a motorcycle after safely dropping the team off, said District Police Officer (DPO) Zahidullah. He added that the two had been shot at by unknown assailants who then managed to escape.

According to the DPO, the two policemen died on the spot. The bodies were shifted to the Mardan District Headquarters Hospital for post-mortem. The DPO further said that the police had launched an operation to search for the assailants and evidence was being collected from the crime scene.

KP Chief Minister Mahmood Khan took notice of the incident, saying the elements involved in the incident will not be able to escape the "grip of the law". He added that the provincial government will take every step to help the martyrs' families.

"The elements involved in the incident are the enemies of our children. The provincial government is committed to rid the province of the poliovirus," he said.

In a tweet, the spokesperson for the provincial government, Kamran Bangash, termed the incident "deeply saddening". He added that KP Chief Minister Mahmood Khan had also expressed grief over the incident and ordered a full investigation.

No one claimed responsibility for the attack, which came two days after the federal government launched a nationwide drive to eradicate the crippling disease by the end of the year.

Militants often target polio teams and police assigned to protect them, claiming the vaccination campaigns are a 'Western conspiracy' to sterilise children.

Pakistan and Afghanistan are the only two remaining countries in the world where polio is endemic, after Nigeria was last year declared free of the virus.

Additional input from AP.