PESHAWAR, Pakistan (AP) — Gunmen on a motorcycle Wednesday shot and killed two police officers assigned to protect polio vaccination workers in northwest Pakistan before fleeing, police said.

The attack happened in the district of Mardan in Khyber Pakhtunkhwa province as the officers returned to a police station after providing security for the polio team, said Adnan Azam, a senior police officer.

No one claimed responsibility for the attack, which came two days after Pakistan launched a nationwide drive to eradicate the crippling disease by the end of the year. Azam said a search for the attackers was underway and all the polio workers were safe.

Pakistani militants often target polio teams and police assigned to protect them claiming the vaccination campaigns are a Western conspiracy to sterilize children.

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