By ZEINA KARAM and PHILIP ISSAnnBEIRUT: Kurdish-led Syrian forces backed by the U.S. 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.nnThe announcement by a coalition of Kurds and Arabs known as the Syria Democratic Forces came at a news conference in Ein Issa, north of Raqqa, attended by senior commanders and representatives of the group, and was immediately welcomed by Washington. 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.nnUnlike 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.nnIn Iraq, the coalition has a coherent government to work with, but the U.S. and its coalition 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 the presence of Russian and Syrian forces on one side and Turkish forces on another.nnStill, the start of the Raqqa offensive increases 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.nnIslamic 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.nnIraq’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.nnKurdish officials at the news conference said the two anti-IS campaigns are not coordinated but simply a matter of “good timing.”nn”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 reading the statement.nnShe said 30,000 fighters will take part in the operation, dubbed “Euphrates Rage,” and that a joint operations command had been set up to coordinate various factions on all fronts.nn”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.nn”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,” Carter added.nnActivists reported clashes Sunday between SDF forces and Islamic State militants north of Raqqa.nnSDF forces seized control of six villages and farms in the countryside, 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.nnRaqqa-based Syrian activist group known as Raqqa is Being Slaughtered Silently also reported the clashes south of Ein Issa.nnBritain 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.nnThe SDF is dominated by the main Syrian Kurdish fighting force known as the People’s Protection Units, or YPG. The United States considers the group to be the most effective force against the IS, but Turkey views it as a terrorist organization and says it’s linked to Turkey’s outlawed Kurdish group. Turkish officials, including President Recep Tayyip Erdogan, have said they will not accept a role for the Kurds in the liberation of Raqqa.nnTurkey’s defense minister last week suggested that instead of the Kurds, Turkish-backed forces can present an “alternative.” In a televised speech Sunday, Erdogan did not comment on the SDF announcement but pointed out that allied Syrian opposition fighters were fast approaching the Syrian town of al-Bab, the last stronghold of the Islamic State group in Aleppo province.nn”Our aim is to go to al-Bab and expel them toward the south,” he said, adding that the opposition fighters were 9 miles (15 kilometers) from the town.nn”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.”nnThere was no immediate comment from the Syrian government of President Bashar Assad on the Kurdish announcement.nnKurdish officials acknowledged their announcement was prompted by Turkey’s growing assertiveness in northern Syria. Starting in August, Ankara has backed Syrian opposition fighters opposed to the Kurds with tanks and aircraft and has bombed Kurdish forces on several occasions.nn”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.”nnThe mutual distrust puts the U.S. in a difficult position as it tries to balance SDF against Turkish interests in northern Syria.nnPolitical 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.nn”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.nnSDF spokesman Talal Sillo told The Associated Press the Raqqa campaign will occur on several fronts.nn”We want to liberate the surrounding countryside, then encircle the city, then we will assault and liberate it,” he said.nnCoalition leaders have been struggling with the timing for the Raqqa campaign, not only because of the demands of the large 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.nnLt. 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.nnTownsend also said the YPG will necessarily be part of the offensive.nn”The facts are these: The only force that is capable on any near-term timeline is the Syrian Democratic Forces, of which the YPG are a significant portion,” he said.nnSource: Associated Press, writers Zeynep Bilginsoy in Istanbul, Turkey, and Gregory Katz in London contributed to this report.