QUETTA, Pakistan (AP) — A pair of gunmen riding on a motorcycle shot and killed a police officer assigned to guard a polio vaccination team in Pakistan's volatile southwestern Baluchistan province on Tuesday, authorities said.

The polio workers escaped unharmed and the two assailants fled the scene of the attack, which took place in the district of Pishin, senior police officer Ghulam Asghar said.

The attack came on the second day of a nationwide anti-polio campaign — the fifth this year — aimed at making Pakistan a polio-free state.

No group immediately claimed responsibility for the attack, but suspicion fell of Islamic militants who often target polio teams and police assigned to protect them, falsely claiming the vaccination campaigns are a Western conspiracy to sterilize children.

Since April, Pakistan has registered 20 new polio cases and the outbreak has been seen as a blow to the country’s efforts to eradicate the disease, which can cause severe paralysis in children.

Quetta is the capital of Baluchistan. For over two decades, the province has been the scene of a low-level insurgency by small Baluch separatist groups demanding independence from Islamabad. Militants also operate in the region, which borders Iran and Afghanistan.