Iraqi soldiers and Shiite militiamen entered the Islamic State-held city of Tikrit on Wednesday, authorities said, breaching one of the biggest strongholds of the extremists in a key test for Iraqi forces.

Explosions and heavy gunfire could be heard as allied Iraqi forces entered the city through its northern Qadisiyya neighborhood, according to video obtained by The Associated Press. Overhead, an attack helicopter fired missiles as soldiers and militiamen laid down heavy machine-gun fire in the neighborhood’s dusty streets as downtown Tikrit loomed in the distance, black smoke rising overhead.

Officials quickly established a supply line to reinforce troops, Salahuddin police Brig. Kheyon Rasheed told the state-run Iraqiyya television.

“The terrorists are seizing the cars of civilians trying to leave the city and they are trying to make a getaway,” Rasheed said. Authorities offered no immediate casualty figures.

A local official in Iraq’s Salahuddin province also confirmed that Iraqi troops and the militias made it into Qadisiyya. He spoke on condition of anonymity as he was not authorized to brief journalists.

The Islamic State group holds about a third of Iraq and neighboring Syria in its self-declared caliphate. Tikrit, the capital of Salahuddin province about 80 miles north of Baghdad, is one of the largest cities held by Islamic State militants and lies on the road connecting Baghdad to Mosul. Retaking it will give Iraqi forces a major supply link to retake Mosul.

U.S. military officials have that said a coordinated military mission to retake Mosul, Iraq’s second-largest city, likely will begin in April or May and involve up to 25,000 Iraqi troops. But the Americans have cautioned that if the Iraqis are not ready, the offensive could be delayed.

Report by The Associated Press