(CNN) US-backed rebels have breached a strategic wall surrounding the Old City of Raqqa in ISIS's self-declared capital on Monday, US Central Command has said in a statement.

Breaching the Rafiqah Wall means the Syrian Democratic Forces will be able to penetrate Raqqa's Old City, the last redoubt of ISIS defenders in the city. The ancient wall -- first constructed in the 8th century by the Abbasid dynasty and stretching around the Old City on three sides -- has provided important fortification for ISIS.

The operation was "a key milestone" in the campaign to "liberate the city," Brett McGurk, the US envoy for the anti-ISIS coalition, said on his official Twitter account.

In a CENTCOM statement, the US added: "Coalition forces supported the SDF advance into the most heavily fortified portion of Raqqa by opening two small gaps in the Rafiqah Wall that surrounds the Old City."

The battle for Raqqa is not dissimilar to that of Mosul, where US-backed Iraqi forces are fighting to expel the last of ISIS fighters from Iraq's second-largest city. But the fight to retake Raqqa has gone quicker, with attacking forces gradually forcing a diminishing number of ISIS fighters into a smaller area of narrow streets around the ancient mosque of Rafiqa, which has already been extensively damaged.

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