BAGHDAD — Iraqi forces pressed deeper into one of the Islamic State’s key strongholds Thursday, further challenging the militants’ hold on the city of Tikrit in a decisive battle that could set the stage for a wider offensive for control of northern Iraq.

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 home town of former strongman 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 seized Tikrit in June as part of a blitz-style advance through northern Iraq.

The gains by Iraqi forces could mark a crucial step in dislodging the Islamic State from other key areas, including the northern city of Mosul.

But it was unclear whether Iraqi forces would hold their ground in Tikrit, which straddles an important highway junction about 110 miles northwest of Baghdad. Government troops have struggled to maintain control over recaptured territory in the past. The Islamic State, meanwhile, has gone on the offensive in other areas, including Ramadi in western Iraq and in the oil-rich territory in eastern Syria.

The head of the military operation told The Associated Press that troops would try to reach the city center on Thursday. The official spoke anonymously, the AP said, as he is not authorized to brief media.

Iraqi television showed civilians in towns near Tikrit greeting pro-government forces as they swept through on their way to the city.

The military offensive includes a combined force of up to 30,000 troops, including a large contingent of Shiite militia fighters.