US military chiefs are grappling with significant intelligence gaps as they bomb Syria and Iraq, according to officials.
The Pentagon is also operating under less restrictive targeting rules than those president Barack Obama imposed on the CIA drone campaign in Pakistan and Yemen.
The US military says its air strikes have been discriminating and effective in disrupting an al Qaida cell called the Khorasan Group and in halting the momentum of Islamic State (IS) militants, but independent analysts say IS remains on the offensive in areas of Syria and Iraq, and still controls large areas.
Human rights groups say coalition air strikes have killed as many as two dozen civilians. US officials say they cannot rule out civilian deaths but have not confirmed any.