Battle escalates as Iraqi forces try to recapture Tikrit from ISIS

The showdown in Tikrit — waged without significant American air support — is also a test of coordination between government-led Sunni forces and Shiite militias backed by Iran. The two groups have often been at odds, but have joined forces against the common enemy of the Islamic State.

Rocket and mortar fire could be heard coming from Tikrit, the hometown of Saddam Hussein, as Iraqi security forces sought to drive back Islamic State fighters in street-by-street combat.

Iraqi troops and allied Shiite militiamen entered Tikrit on Wednesday on two fronts, pushing through bomb-rigged defenses set up by the Islamic State — which took hold of Tikrit in June as part of a blitz-style advance through northern Iraq.