The Associated Press
This frame grab from video provided by Arab 24 network, shows officials with the U.S.-backed Syria Democratic Forces at a press conference in Ein Issa in northern Syria. U.S.-backed Kurdish-led Syrian forces announced their plan Sunday to retake the Islamic State group's de facto capital of Raqqa, saying they hoped Turkey would not "interfere in internal Syrian affairs." (Arab 24 network, via AP)

BEIRUT — Backed by U.S. airstrikes, Kurdish-led Syrian fighters clashed on Monday with Islamic State militants north of the city of Raqqa in Syria as they pushed ahead in their offensive aiming to liberate 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 IS fighters.

The U.S. commander of coalition forces fighting the Islamic State group said the operation to liberate Raqqa aims at eventually cutting off the extremists from the Iraqi city of Mosul, where IS militants are already under attack by U.S.-backed Iraqi forces.

Lt. Gen. Townsend said in a statement 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 IS tactical units and destroyed six fighting positions, five vehicles and two vehicles rigged with explosives. The command said the airstrikes were conducted on Sunday.

The Britain-based Syrian Observatory for Human Rights said heavy fighting was underway on Monday north of Raqqa between IS 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.

The IS-linked Aamaq news agency said IS militants hit an SDF armored vehicle north of Raqqa with a missile, killing everyone inside.

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.