BEIRUT >> Kurdish-led Syrian forces began an offensive Sunday to liberate the Islamic State group’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 IS 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 group, making it harder for its fighters to move reinforcements between Syria and Iraq. The city, which has been under IS control since early 2014, is home to some of the group’s top leaders and is seen as the key to defeating the group 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 IS 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 (48 kilometers) north of Raqqa. The Kurdish officials said the anti-IS campaigns in Mosul and Raqqa are not coordinated but simply a matter of “good timing.”

“We call on our heroic steadfast people in Raqqa and surrounding areas to stay away from enemy gatherings which will be a target for the liberating forces and the coalition forces, and to head to areas that will be liberated,” said Cihan Ehmed, an SDF fighter.

She 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.

“I welcome today’s announcement by the SDF that the operation to free Raqqa from ISIL’s barbaric grip has begun,” U.S. Defense Secretary Ash Carter said. “The effort to isolate and ultimately liberate Raqqa marks the next step in our coalition campaign plan.

“As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of ISIL’s caliphate and disrupt the group’s ability to carry out terror attacks against the United States, our allies and our partners,” he added.

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

SDF forces seized control of six villages and farms, the Britain-based Syrian Observatory for Human Rights said. It also reported strong activity by U.S.-led coalition warplanes and airstrikes that hit IS positions. The Observatory also said IS detonated two car bombs targeting the advancing forces.

The Raqqa-based Syrian activist group known as Raqqa is Being Slaughtered Silently also reported the clashes south of Ein Issa.

There was no immediate comment from the government of Syrian President Bashar Assad on the Kurdish announcement.

Brett McGurk, the White House envoy to the anti-IS coalition, said the U.S. will provide air support for the Raqqa offensive and is in “close, close contact” with its ally Turkey. “We want this to be as coordinated as possible, recognizing that there will be a mix of forces on the field,” he told reporters in Jordan.

Britain also said it is providing aerial surveillance to help the offensive. The Royal Air Force “will support the Raqqa operation as it develops,” Defense Secretary Michael Fallon said.

French Defense Minister Jean-Yves Le Drian said his country will provide airstrikes to aid the offensive. He told French radio Europe 1 that “local territorial forces” should retake Raqqa with air support from the coalition but no foreign ground troops.

The SDF is dominated by the main Syrian Kurdish fighting force known as the People’s Protection Units, or YPG. The U.S. considers the group to be the most effective force against IS, but Turkey views it as a terrorist organization linked to a Kurdish group outlawed by the government in Ankara. Turkish officials, including President Recep Tayyip Erdogan, have said they will not accept a role for the Kurds in the liberation of Raqqa.

Turkey’s defense minister suggested last week that instead of the Kurds, Turkish-backed forces can present an “alternative.”

In a speech Sunday, Erdogan did not comment on the SDF announcement but said allied Syrian opposition fighters were 9 miles (15 kilometers) from the Syrian town of al-Bab, the last stronghold of the IS group in Aleppo province. He added that Turkey’s aim was to move on al-Bab and expel the militants toward the south.

“Our hope is that the Turkish state will not interfere in the internal affairs of Syria,” Ahmad said at the news conference, suggesting SDF forces would defend themselves if it did. “Raqqa will be free by its own sons.”

Kurdish officials acknowledged their announcement was prompted by Turkey’s growing assertiveness in northern Syria. Since August, Ankara has backed Syrian fighters opposed to the Kurds with tanks and aircraft and has bombed Kurdish forces.

“Let’s be clear: Turkey is an enemy of the Kurds,” said Nawaf Khalil, the former spokesman for Syria’s powerful Kurdish Democratic Union Party, known as the PYD. “When Turkey enters an area, it remains as an occupier.”

The mutual distrust puts the U.S. in a difficult position as it tries to balance SDF against Turkish interests in northern Syria.

Political official Rezan Hiddo said the SDF had warned the international coalition it would halt its Raqqa campaign if Turkish-backed forces advanced on Manbij or other Kurdish-held towns.

“We cannot extinguish the fire in our neighbor’s house if our home is burning. We were very clear with our allies. If there is a plan to attack Daesh, there must be limits for Turkey,” Hiddo said, using an Arabic acronym for the Islamic State group.

SDF spokesman Talal Sillo told The Associated Press the Raqqa campaign will occur on several fronts.

“We want to liberate the surrounding countryside, then encircle the city, then we will assault and liberate it,” he said.

Coalition leaders have been struggling with the timing of the Raqqa campaign, not only because of the demands of the Iraqi-led Mosul operation but also because the political and military landscape in Syria is more complicated amid a civil war that has lasted more than five years and has devastated much of the country.

Lt. Gen. Stephen Townsend, commander of anti-IS coalition forces, said last week more Syrian opposition fighters need to be recruited, trained and equipped for the Raqqa battle. The YPG will necessarily be part of the offensive, he said, adding: “The only force that is capable on any near-term timeline is the Syrian Democratic Forces, of which the YPG are a significant portion.”