A Syrian Democratic Forces fighter stands near the rubble of a building north of Raqqa, Syria, on Nov. 7, 2016. (Rodi Said/Reuters)

Islamic State fighters launched a wave of car bombs in northern Syria on Monday as U.S.-backed forces made the first moves in a long-awaited push to retake the militants’ de facto capital.

The Kurdish-led military operation, announced Sunday, seeks to isolate the city of Raqqa in preparation for an assault that could deal a devastating blow to the Islamic State — which is also fighting Iraqi-led forces in the northern Iraqi city of Mosul.

The two cities represent critical hubs for the Islamic State, and the twin offensives could cut off routes for supplies and reinforcements. In a rare audio recording released Thursday, the Islamic State’s leader, Abu Bakr al-Baghdadi, urged militants to fight to the death in Mosul.

The rush of car bombs that met the alliance, known as the Syrian Democratic Forces (SDF), offered an early taste of how ferocious the battle for Raqqa may become. The blasts occurred as the forces pushed south from the towns of Ain Issa and Suluk, about 30 miles north of Raqqa.

Jihan Sheikh Ahmed, a spokeswoman for the SDS, said the Islamic State had sent seven car bombs toward the approaching forces.

The Islamic State has become increasingly reliant on suicide attacks to slow the progress of the array of forces rolling back its territory. The Amaq news agency, the extremist group’s media arm, claimed that the Islamic State launched 120 such assaults across Syria, Iraq and Libya in October.  

The U.S.-led coalition fighting the Islamic State said Monday that it had begun airstrikes in support of the offensive near Ain Issa, destroying six of the militants’ fighting positions and seven vehicles, two of them packed with explosives.

At the same time, the United States was seeking to calm NATO ally Turkey, which is uneasy over any increased influence by Kurdish groups in the region. Ankara has waged battles against Kurdish factions in southeastern Turkey for decades.

On a visit to Turkey’s capital, Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, cautioned that retaking Raqqa would be neither swift nor simple.

“We always advertised that the isolation phase is going to take months,” he told an accompanying Defense Department reporter Sunday.

The Obama administration faces a delicate balancing act between Turkey and the Syrian Kurdish forces the United States has trained and equipped. Ankara draws no distinction between these militants and the Kurdish insurgents that launch regular attacks on Turkish soil.

U.S. defense officials said that they are still hammering out details of the planned assault into Raqqa but that they do not envision a direct role for Turkey in that operation. Officials have said the NATO ally could help block militant movements out of the Raqqa area or focus on securing the Turkish border.

Talks this weekend between Dunford and Turkey’s top military officer, Gen. Hulusi Akar, were aimed at securing Turkish support for the Pentagon’s concept of the operation and at reassuring Ankara that the offensive into Raqqa, which U.S. officials say will be spearheaded by Arab forces, would not hand additional territory to Syrian Kurds.

Missy Ryan in Washington and Zakaria Zakaria in Istanbul contributed to this report.