Gunmen on motorcycles opened fire on Pakistani police escorting a team of polio workers on Tuesday during a door-to-door vaccination campaign in the southwest, killing two police officers before fleeing, police said.

The polio workers were unharmed in the attack on the outskirts of Quetta, the capital of Baluchistan province, local police chief Asif Marwat said.

No one has claimed responsibility, but Pakistan’s anti-polio campaigns are regularly marked by violence. Islamic militants often target polio teams and police assigned to protect them, falsely claiming that the vaccination campaigns are a Western conspiracy to sterilize children.

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Since January, Pakistan has reported only one polio case, raising hopes that the country is close to eradicating the disease.

Pakistan and Afghanistan are the last two countries in which polio has not been eliminated.