BEIRUT/ASTANA: U.S.-backed forces have penetrated the heavily fortified heart of militant bastion Raqqa for the first time, in a key milestone in the war against Daesh (ISIS) in Syria.

Meanwhile, at the Astana peace talks that opened Tuesday, Russia said it might deploy its military to police the borders of planned de-escalation zones in Syria within two to three weeks after finalizing an agreement with Turkey and Iran.

Airstrikes by the U.S.-led coalition battling Daesh punched two holes in the medieval wall surrounding Raqqa’s Old City, allowing fighters of the Syrian Democratic Forces to breach the militants’ defenses, Washington and the SDF said Tuesday.

The advance – the culmination of a nearly 8-month-old campaign – comes as the militants face an expected defeat within days in Iraq’s second city Mosul, the other pivot of the cross-border “caliphate” they declared in 2014.

The SDF said coalition warplanes opened up two breaches in the 2.5-kilometer Rafiqah Wall around Raqqa’s Old City, enabling its fighters to evade explosives laid by Daesh.

“Daesh have used this archaeological wall to launch attacks, and planted bombs and mines in its gates to hinder the advance of SDF forces,” the alliance said.

“There have been fierce clashes [in the Old City] since dawn today, with 200 of our fighters mobilizing to the area,” said Mohammad Khaled Shaker, a spokesman for the Syrian Elite Forces, U.S.-backed Arab fighters allied with the SDF.

The Syrian Observatory for Human Rights activist group said U.S.-backed forces “are clashing with [Daesh] at four points in the eastern part of the Old City” as airstrikes pummeled other neighborhoods.

The U.S. envoy to the coalition, Brett McGurk, said on Twitter that breaching the Rafiqah Wall marked a “key milestone in [the] campaign to liberate the city.”

U.S. Central Command, which oversees military operations across the Middle East, said the coalition airstrike had allowed advancing forces “to breach the Old City at locations of their choosing.”

This prevented Daesh from using booby traps, land mines and suicide car bombs, “protected SDF and civilian lives, and preserved the integrity of the greatest portion of the wall,” it said.

The Rafiqah Wall, which surrounds the city’s historic heart, originally dates back to the late eighth century, when as capital of the Abbasid caliphate, Raqqa was briefly the center of the Islamic world.

The Observatory said the advance had been supported by U.S. Special Forces and constituted the “most important progress” yet for the SDF, which broke into Raqqa on June 6.

But Daesh still controls 70 percent of the city, according to the Observatory, and the toughest battles are yet to come.

“Today, the real fight for Raqqa has begun. There are both many civilians and many mines in the Old City,” Observatory chief Rami Abdel-Rahman said. As Daesh loses ground, tensions are rising among the array of forces battling it. Turkey shelled several villages in Syria overnight Tuesday, 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.

Elsewhere, power brokers Russia, Iran and Turkey focused on shoring up a plan for safe zones in Syria at a fifth round of talks they are pushing in Kazakhstan to help end the 6-year-old conflict.

Russian negotiator Alexander Lavrentyev told reporters that Moscow and its partners were still discussing detailed maps and other conditions related to the Idlib and southern zones, while the borders of two other zones, in Homs province and near Damascus, had been agreed.

“Overall, [the agreement] provides for the presence of Russian military police in the buffer zones, but once again this matter has not been agreed yet,” he said. “Depending on when the documents on safe zones are signed, I think one should expect concrete measures on the deployment of forces within two to three weeks.”

The Astana talks received a boost Monday after the Syrian army unilaterally announced a halt to fighting until midnight on July 6 in the southern Deraa, Qunaitra and Swaida provinces, which together cover one of the zones. Deraa had seen the fiercest fighting in the areas envisioned as safe zones in recent weeks.

But an AFP correspondent said airstrikes pummeled the key opposition-held town of Douma in a rebel enclave of Eastern Ghouta just outside Damascus Tuesday for the first time since the de-escalation zones were announced in May.

A medical source told AFP a woman and a child were killed in the raid, a toll confirmed by the Observatory, which also said that 10 others were badly wounded. The Observatory said the raids were carried out by Syrian warplanes.