Syrian Democratic Forces commanders announce the impending assault on the Islamic State. (Rodi Said/Reuters)

A Kurdish-led force backed by the United States announced on Sunday the start of a major military operation to drive Islamic State militants out of their self-declared capital of Raqqa, in northeastern Syria.

The operation by the Syrian Democratic Forces, or the SDF, is timed to coincide with the U.S.-supported military effort to seize the Iraqi city of Mosul from the Islamic State. The assault by the SDF — an alliance of Kurdish and Arab forces that has dealt substantial blows to the Islamic State in northeastern Syria — represents an intensified international effort to squeeze the extremist group as it loses control of vast territory in Iraq and Syria.

The Islamic State has been badly weakened by airstrikes that have killed its leaders and destroyed its infrastructure, as well as by ground assaults from an array of U.S.-backed forces.

Those ground attacks, carried out by Kurds and Arabs in Iraq and Syria, have driven the militants out of key strongholds, such as the Iraqi city of Fallujah, and towns along the Syrian border with Turkey that had been used as hubs for trade and for funneling fighters and weapons.

Officials in the U.S.-led coalition against the Islamic State say that defeating the group in its two most important cities, Raqqa and Mosul, could deal a devastating blow to the militants.

After making sweeping advances in Iraq two years ago, the Islamic State, also known as ISIS and ISIL, declared a caliphate, imposing harsh religious rule based on an extreme interpretation of Islam.

“The initial phase of operations to liberate Raqqa has begun,” Brett McGurk, the State Department’s envoy to the coalition against the Islamic State, said at a news conference in Jordan’s capital, Amman.

He said the SDF — backed by U.S. Special Forces playing an advisory role in Syria — aims to attack in phases to eventually “isolate” the Islamic State in Raqqa and stop the flow of militants in and out of the city, about 275 miles northeast of the Syrian capital, Damascus.

But the operation, called Euphrates Rage, adds yet another potentially combustible wrinkle to the five-year-old Syrian conflict.

Already, Kurdish efforts to exploit the chaos and build an autonomous region in northern Syria have aggravated the country’s sectarian politics — with some U.S.-allied Syrian rebels opposed to the Kurdish moves — and inflamed regional tensions.

Turkey, in particular, views with great suspicion the leadership role of the Kurdish People’s Protection Units, or the YPG, in the Syrian Democratic Forces. Over the summer, Turkish forces intervened in northern Syria, targeting Islamic State militants but also acting as a curb to Kurdish territorial ambitions in the country.

At a news conference in the eastern Syrian town of Ain Issa, SDF officials said Sunday that as many as 30,000 fighters would participate in the Raqqa operation. The U.S.-led coalition provides the SDF with training, arms and air support.

At the news conference, an unnamed SDF official expressed concern about potential Turkish involvement in the assault.

“Our hope is that the Turkish state will not interfere in the internal affairs of Syria,” the unidentified SDF official said, according to Reuters.

Last week, Turkey’s defense minister said his country’s forces could lead the attack on Raqqa instead of the SDF. Even though the United States considers the Syrian force to be the most effective in battling the Islamic State, Turkey sees the involvement of YPG militants as a threat.

Turkey, part of the North Atlantic Treaty Organization and a key U.S. ally, considers the YPG a terrorist group because of its links to Turkey’s own Kurdish separatists.

Nevertheless, U.S. officials have indicated that the SDF would lead the Raqqa operation. Last month, Defense Secretary Ashton B. Carter said a military operation targeting the city should coincide with the attack on Mosul.

By Sunday afternoon, SDF fighters did not appear to have made any major advances, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group. “It has not formally started yet. This is just an announcement,” said Rami Abdulrahman, a pseudonym for the director of the group.

It will probably take weeks, if not longer, before fighters enter Raqqa. SDF fighters expect to face intense resistance similar to that encountered by advancing Iraqi forces on the outskirts of Mosul. The Islamic State has had ample time to dig in its heels since seizing Raqqa in early 2014, and its fighters are likely to use tunnels for ambushes, booby-trap vehicles and suicide bombers.

Zakaria Zakaria in Istanbul and Taylor Luck in Amman contributed to this report.