Story highlights Police killed in drive-by shootings

The officers were killed in two incidents, police say

Pakistan Taliban offshoot claim responsibility for the killings

(CNN) Seven police officers were killed while protecting health workers who were vaccinating children against polio in the Pakistani city of Karachi, authorities said.

Eight militants on four motorcycles opened fire on officers who had stopped for food and then shot at a convoy protecting the health workers, police inspector general Feroz Shah said.

Speaking at a news conference, Ad Khawaja, the police chief for Sindh Province, said authorities had identified the group responsible for the twin attacks.

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Shell casings found at the two crime scenes matched those from previous attacks.

Pakistani Prime Minister Muhammad Nawaz Sharif released a statement condemning the attacks.