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U.S.-backed fighters in Syria have seized the ISIS stronghold of Raqqa after months of battle, a militia commander said Tuesday.

"Raqqa is completely under control," said Cmdr. Lt. Gen. Talal Silo, a spokesman for the Syrian Democratic Forces (SDF), which are battling the hyper-violent jihadis. He added that they were slowly searching the city for any remaining ISIS fighters, which could take several days.

SDF fighters celebrated their victory Tuesday, waving and raising flags throughout the city, and gathering at al-Naim Square where ISIS once carried out executions.

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The fall of ISIS' de facto capital is a significant military victory as well as a symbolic one.

The militias have battled for control since June in a painstaking block-by-block, and sometimes building-by-building, fight for control.

The intense fighting has left the city battered, with the the London-based Syrian Observatory for Human Rights saying that around 80 percent of it had been destroyed.

Related: Fall of Raqqa Will Change War on Extremists, Not End It

Raqqa's official population once stood at around 220,000 — around the size of Richmond, Virginia. It is now a shadow of its former self. The International Committee of the Red Cross estimates that between 20,000 and 30,000 people fled Raqqa in the past year alone.

The charity Save the Children said that approximately 270,000 people have been displaced from Raqqa and the surrounding areas, and a humanitarian crisis is now brewing with refugee camps bursting at the seams. It will be months, if not years, before many of the people will be able to return to their homes, the charity said.

Photos: The Battle for Raqqa

At the height of its power, ISIS ruled over millions of people and attracted thousands of overseas fighters. From Raqqa, located in northern of Syria on the Euphrates River, it oversaw controlled much of eastern, central and northern Syria.

It now holds on to fraction of its former territory.

Charlene Gubash reported from Cairo. Rachel Elbaum reported from London.