BEIRUT — Kurdish-led Syrian forces began an offensive Sunday to liberate the Islamic State’s de facto capital of Raqqa, clashing with the extremists north of the Syrian city and warning neighboring Turkey not to interfere in the operation.

The United States, France and Britain said they would provide air support for the offensive, which was announced at a news conference in Ein Issa, north of Raqqa, by a coalition of Kurds and Arabs known as the Syria Democratic Forces.

But it lacked details on how the group 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 fighters.

Unlike several successful military efforts to drive Islamic State militants out of cities in Iraq, the Raqqa offensive faces several political obstacles and is likely to be much more complex.

In Iraq, a U.S.-led coalition is working with the government in Baghdad, but Washington and its partners in Syria are relying on a hodgepodge of local Arab and Kurdish opposition groups, some of which are fierce rivals. The tensions are exacerbated by Russian and Syrian forces on one side and Turkish forces on another.

Still, the start of the Raqqa offensive, which aims initially at isolating and encircling the city, increases the pressure on the Islamic State, making it harder for its fighters to move reinforcements between Syria and Iraq. The city, which has been under Islamic State control since early 2014, is home to some of its top leaders and is seen as the key to defeating the extremists militarily.

Islamic State forces already are now under attack by U.S.-backed Iraqi forces on the eastern edges of the city of Mosul, which the militant group seized two years ago when it captured territory across Iraq and Syria for its self-proclaimed caliphate. The Iraqi forces began their operation Oct. 17 and are trying to push deeper into the city, which is the militants’ last urban bastion in Iraq.

Iraq’s Hezbollah Brigades, one of the main Shiite militias taking part in the government-led push to drive the Islamic State from Mosul, said Wednesday its men had taken control of a highway linking Mosul and Raqqa, severing a key supply route between the two militant strongholds.

Senior commanders and representatives of the SDF attended the news conference in Ein Issa, about 30 miles north of Raqqa. The Kurdish officials said the anti-Islamic State campaigns in Mosul and Raqqa are not coordinated but simply a matter of “good timing.”

Activists reported clashes Sunday between Islamic State militants and SDF forces north of Raqqa.