BEIRUT (AP) — U.S.-backed Syrian forces have breached the wall around Raqqa's Old City, the U.S. military said Tuesday, marking a major advance in the weeks-old battle to drive Islamic State militants out of their self-declared capital.

The U.S. Central Command said the coalition conducted targeted strikes on two "small portions" of the Rafiqah Wall, allowing the fighters "to advance into the most heavily fortified portion" of the city. It said the strikes allowed SDF fighters to bypass booby traps and snipers, and left most of the 2,500-meter (yard) wall intact.

The head of the Britain-based Syrian Observatory for Human Rights, Rami Abdurrahman, said the breaching of the wall was the most important development to date in the battle for Raqqa. He said three SDF units advanced toward the wall under air cover, breaking through the IS defenses, and that heavy clashes were underway.

The Kurdish-led Syrian Democratic Forces launched a multi-pronged assault on Raqqa last month after securing the surrounding countryside. On Sunday, the U.S.-backed fighters crossed the Euphrates River on the southern edge of the city, completing its encirclement.

The U.S.-led coalition is providing close air support to the SDF, which has already driven the extremists from much of northern and eastern Syria.

Several IS leaders were once based in Raqqa, where the group is believed to have plotted attacks in Europe. The loss of the northern Syrian city, one of the last IS strongholds, would deal a major blow to the group. The militants are also on the verge of losing their last foothold in the northern Iraqi city of Mosul, where they declared an Islamic caliphate three years ago.