BEIRUT — The battle for control of the Islamic State’s de facto capital Raqqa, in northern Syria, will begin “within days,” a spokeswoman for a U.S.-backed Syrian force at the city’s edge said Saturday.

The Kurdish-led Syrian Democratic Forces are already encamped around the city’s northern and eastern divisions and Saturday made new progress against the militants to approach the city from the south bank of the Euphrates River. Raqqa lies on the northern side of the river.

Spokeswoman Cihan Sheikh Ehmed said the SDF, which enjoys the backing of the U.S. military, would launch the battle for Raqqa “very soon.”

The Britain-based Syrian Observatory for Human Rights said the SDF has been engaged in fierce fighting with militants along the southern bank of the Euphrates River, around Mansoura, about 16 miles southwest of Raqqa. The SDF said they were in control of 90 percent of the town Saturday.

Raqqa’s size poses a new challenge for the SDF, who have captured smaller towns and strongholds from the Islamic State in northern Syria. As of March, there were an estimated 300,000 people inside Raqqa. Activists reported the militants were forcing them to stay and using them as human shields.

Raqqa is the largest city to have fallen under the complete control of the Islamic State in Syria, after militants seized it from rebels in January 2014. The city’s capture heralded nearly two years of dizzying expansion for the Islamic State across northern Syria and Iraq, and it formally split with al-Qaida’s central leadership in February that year. Shortly after, its leader declared a “caliphate” over the areas under Islamic State control, which stretched to include Mosul, Iraq’s second largest city. That designation is not recognized by other Muslims.

The Islamic State is also struggling to defend Mosul, the largest city it once held in neighboring Iraq. U.S.-backed Iraqi forces and allied militias have captured most of Mosul since launching their campaign last October.

On Wednesday, the Islamic State’s Aamaq news agency reported the coalition had destroyed Raqqa’s main telecommunication’s center.

The campaign has led to wide-scale displacement around the Raqqa province, according to the U.N, and conditions are deteriorating inside the provincial capital.

There are also reports of mounting civilian casualties, though they are difficult to confirm because of the war environment.

In May alone, nearly 95,000 residents fled their homes or shelters because of violence in the Raqqa province, according to the U.N.’s refugee agency, UNHCR. But others have returned to their homes as the SDF captures Islamic State-held ground.

“The offensive on Raqqa has intensified over recent days, when more than 100 air and artillery strikes were reported to have caused many civilian casualties,” the agency said in a June 1 report.

The violence around Mansoura has produced conflicting casualty allegations. An airstrike leveled a school in the town on March 21, leading local monitoring groups to say at least 33 civilians taking shelter inside had been killed.

Lt. Gen. Stephen Townsend acknowledged days later that coalition aircraft bombed the school but said preliminary intelligence indicated the victims were Islamic State militants occupying the building, not refugees.

Roadblocks and damage to bridges and infrastructure has driven up the prices of fuel and basic foodstuffs inside Raqqa, according to the U.N.’s humanitarian agency OCHA, compounding the hardship.

The U.S. has backed the SDF with weapons, airpower and ground support in its campaign to defeat the Islamic State in northern Syria. It began arming the fighters under a new order from the Trump administration in late May, to the dismay of Turkey, which says the factions receiving the weapons are terrorists affiliated with the Kurdish insurgency inside its own borders.