Attack in Karak, a town in Khyber Pakhtunkhwa province, marks the second day of a five-day national immunisation drive.
Gunmen riding a motorcycle have shot dead two police officers assigned to protect polio vaccination workers in northwest Pakistan, according to police.
The attack took place on Wednesday in the district of Mardan in Khyber Pakhtunkhwa province as the officers returned to a police station after providing security for the polio team.
The policemen died at the scene, while the attackers fled. All the polio workers were safe. Police said they have launched a manhunt to find the perpetrators.
There was no immediate claim of 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.
The weeklong effort aims to vaccinate tens of millions of children below the age of five with the help of some 260,000 front-line workers.
Pakistani fighters regularly stage attacks on polio teams and police escorting them, claiming the anti-polio drive is part of a Western conspiracy.
These attacks increased after it was revealed that a fake hepatitis vaccination campaign was used as a ruse by the CIA in the hunt for al-Qaeda leader Osama bin Laden, who was killed by US commandos in 2011 in Pakistan.
In January, a policeman was killed in a similar drive-by shooting in the province, while in March, gunmen killed three women involved in a polio immunisation programme across the border.
The challenges faced by polio teams have sparked fears that any drive to inoculate the population against the coronavirus will also run into problems.
Pakistan has so far reported some 936,100 COVID-19 cases and more than 21,000 deaths.
The country and neighbouring Afghanistan are the only two in the world where polio is still endemic after Nigeria was last year declared free of the virus.
Pakistan, home to 220 million people, has so far reported only one polio case this year, but the number was a high of 147 in 2019 before dropping to 84 last year.