BEIRUT -- Backed by U.S. airstrikes, Kurdish-led Syrian fighters clashed on Monday with ISIS militants north of the city of Raqqa in Syria as they pushed ahead in their offensive aiming to retake the city that has been the de facto capital of the extremist group since 2014.

Members of a coalition of Kurdish and Arab fighters known as the Syria Democratic Forces have been on the offensive in the area since late Saturday, aiming to initially isolate and encircle Raqqa.

The United States, France and Britain said they would provide air support for the offensive, which was announced at a news conference on Sunday in Ein Issa, north of Raqqa.

But the announcement lacked details on how the SDF, dominated by Kurds, plans to oust the militants from the city, home to nearly 200,000 mostly Sunni Arabs and an estimated 5,000 Islamic State of Iraq and Syria (ISIS) fighters.

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The battle of Raqqa could be long and costly; ISIS fighters are expected to fight until the end since its loss could be an existential threat. It would mean the extremist group would have no full control of any large cities in Syria, just as Iraqi forces are pushing forward against ISIS-held Mosul, Iraq’s second largest city.

Earlier this year, it took SDF fighters more than two months to capture the Syrian town of Manbij, which is far smaller than Raqqa.

Raqqa has been under ISIS control since early 2014; it is home to some of the group’s top leaders and is seen as the key to defeating the group militarily. It has been the extremists’ de facto capital since they declared a caliphate in areas they control in Iraq and Syria in June 2014.

The U.S. commander of coalition forces fighting ISIS said the operation to take Raqqa aims at eventually cutting off the extremists from Mosul, where U.S.-backed Iraqi forces have entered the city’s eastern outskirts amid fierce resistance.

Lt. Gen. Stephen J. Townsend said in a statement that the Arab element of the SDF is “indigenous to the area” and will help establish “regional support” for SDF operations. His comments appeared to be aimed at soothing concerns that Kurdish forces would take over Raqqa, a predominantly Sunni Arab city.

The fighting in northern Syria is taking place under the cover of airstrikes by the U.S.-led coalition that played an instrumental role in the past in battles between SDF and the extremists.

The U.S. Central Command said coalition aircraft conducted 16 airstrikes in northern Syria, mostly near the area of Ein Isssa, north of Raqqa, where the fighting appeared to be concentrated. It said that they engaged 12 ISIS tactical units and destroyed six fighting positions, five vehicles and two vehicles rigged with explosives. The command said the airstrikes were conducted on Sunday.

On Saturday, SDF fighter Cihan Ehmed said 30,000 fighters will take part in the offensive, dubbed “Euphrates Rage,” and that a joint operations command had been set up to coordinate various factions.

The U.S.-backed SDF is made up of several Arab, Kurdish, Turkmen and Christian groups that have captured wide areas of northern Syria from ISIS over the past year. The largest and most powerful groups in the coalition are the main Kurdish militias known as the People’s Protection Units, or YPG, and the Women’s Protection Units, or YPJ.

Fayad al-Ghanem, the commander of the small Arab Raqqa Hawks Brigade group, said in a video posted online that their aim is to besiege the city of Raqqa and liberate villages and towns around it.

“We want to rescue the people from the injustice of Daesh,” al-Ghanem said using an Arabic acronym to refer to ISIS.

The Britain-based Syrian Observatory for Human Rights said heavy fighting was underway on Monday north of Raqqa between ISIS and SDF fighters. A Kurdish media official in the area, Rania Mohammed, said several farms and villages have been captured since Sunday, without providing further details.

SDF said its fighters destroyed a vehicle rigged with explosives in a farm north of Raqqa.

ISIS’ own Aamaq news agency said the group’s militants hit an SDF armored vehicle north of Raqqa with a missile, killing everyone inside. It later reported that ISIS fighters destroyed six SUVs with missiles, killing seven SDF fighters.

The Raqqa-based Syrian activist group known as Raqqa is Being Slaughtered Silently reported that SDF fighters shelled the village of Tweila’a north of Raqqa forcing its residents to flee. It also reported that warplanes of the U.S.-led coalition destroyed a bridge in the village.