Raqqa, Syria (CNN)CNN has gained rare access inside the Old City of Raqqa, as rebels and international forces tightened their circle around ISIS' de facto capital.
CNN exclusive: Inside ISIS-held Raqqa
US-led airstrikes breached the 1,300-year-old Rafiqah Wall on Monday and allowed the US-backed Syrian Democratic Forces (SDF) to enter the city from the south. CNN is the first Western media organization to film inside the fortification since then.
After pushing through one of two small gaps in the wall, CNN journalists met with the rebels leading the assault just 200 meters from ISIS positions, as they heard gunfire and explosions nearby.
The rebels have seized control of territory around 300 meters into the Old City since Monday, but have been confronted with fierce resistance from ISIS.
CNN saw US forces in this area and it appears they have been calling in the airstrikes that have cleared the way for the rebels' advances. US personnel did not want to be filmed.
The battle for Raqqa will be the most significant phase in the fight against ISIS to date and it is expected to take months. ISIS has held the city since 2014 and considers it the heart of its envisaged caliphate.
Reporters have had little access to Raqqa, and occasional satellite images and smuggled videos have provided the few glimpses into one of the world's most isolated cities.
For years, phones and cameras were banned and anyone caught with videos or images would have faced death under ISIS' brutal regime.
The SDF has encountered significant ISIS opposition since entering the city on Monday, a US defense official told CNN. Other US officials say overhead imagery shows ISIS has laid booby traps and improvised explosive devices, as it has in other battles. There is now concern that the fighters may use civilians as human shields to defend the city.
The international coalition has nevertheless begun planning for an internal security force of local recruits that could "hold" Raqqa once military operations are over.
"We're already starting to work on something that's called the Raqqa Internal Security Force," Canadian Armed Forces Brigadier General D.J. Anderson told Pentagon reporters.
"This will be locals that will work for the Raqqa Council. So it'll work for local governance, and they'll be ready to establish that policing function, if you will, that safety and security element."
Retaking Raqqa will be a significant blow to ISIS, but it may not mark the group's demise in Syria. US officials have long said they have intelligence showing hundreds of ISIS fighters have fled Raqqa in recent months into the Euphrates River Valley, including some top leaders.