BEIRUT >> U.S.-backed forces in Syria have breached the wall around Raqqa’s Old City, the U.S. military said today, 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 struck two “small portions” of the Rafiqah Wall, allowing the Kurdish-led Syrian Democratic Forces “to advance into the most heavily fortified portion” of the city, bypassing booby traps and snipers. It said the strikes left most of the 2,500-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.

Footage provided by the SDF showed their fighters roaming Qasr al-Banat, a historic quarter inside Raqqa’s Old City. Another unit entered through the so-called Baghdad Gate, opening up a second front inside the Old City.

Brett McGurk, the top U.S. envoy for the international coalition against the Islamic State group, hailed the breach, saying it was a “key milestone” in the campaign to seize the IS stronghold.

The U.S. military said IS fighters were using the historic wall as a fighting position, and had planted explosives at several openings. It said coalition forces were making every effort to protect civilians and preserve the historic sites.

The Kurdish-led Syrian Democratic Forces launched a multi-pronged assault on Raqqa in early June, 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 Islamic State group seized Raqqa, their first major city stronghold in Syria, in January 2014. The city later became the de facto capital of IS’ self-proclaimed caliphate, stretching across lands controlled by the militant group in Syria and Iraq.

U.N. officials say 50,000 to 100,000 civilians remain in the city amid “dire” conditions. Those who try to escape risk being attacked by IS militants or forcibly recruited as human shields.

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 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, from where IS leader Abu Bakr al-Baghdadi proclaimed the IS caliphate in July 2014.

As IS loses ground, tensions are rising among the array of forces battling it.

Turkey shelled several villages in Syria overnight, killing a woman and two children, according to Kurdish officials and Syrian activists. The Kurdish-run Hawar news agency said the three were killed, and several others wounded, near Afrin, a Kurdish-controlled enclave near the border with Turkey.

Turkey’s private Dogan news agency said Turkish artillery units responded after the border region came under fire late Monday.

The SDF is dominated by the People’s Protection Units, or YPG, which Turkey views as an extension of the Kurdish rebels fighting in its southeast.

Turkish Defense Minister Fikri Isik said today that Turkey may launch a cross-border operation into Afrin if it constitutes a “constant security threat.”