BEIRUT — A Kurdish-led force backed by U.S. airstrikes launched an offensive Tuesday seeking to capture territory around the northern Syrian city of Raqqa, in the first ground attack to directly challenge the Islamic State's control of its self-proclaimed capital.

A few thousand Kurdish and Arab fighters — grouped under the umbrella of the U.S.-backed Syrian Democratic Forces — began moving south through the desert from the existing front line about 30 miles north of Raqqa, according to a statement from the SDF and the U.S. military.

The operation aims to secure control of a stretch of territory north of Raqqa, said U.S. military spokesman Col. Steve Warren, speaking by telephone from Baghdad.

He described the operation as a significant step toward an eventual bid to recapture Raqqa, and part of wider pressures on the militants on multiple fronts in Syria and Iraq.

“Certainly they're not going to seize Raqqa with this offensive, but they are going to squeeze Raqqa directly,” he said. “This is putting direct pressure on Raqqa itself. The enemy will be forced to react.”

In recent days, there had been signs of growing panic within Islamic State ranks as indications mounted that some operation was planned for Raqqa, whose loss would be a major blow to the militants.

Warplanes with the U.S.-led coalition dropped leaflets over the city late last week urging civilians to flee, and on Friday the Islamic State lifted a monthslong ban on all travel out of Raqqa.

A statement by the Islamic State's chief spokesman on Saturday appeared to acknowledge unease among residents of the city, according to Ahmed Mhidi of the Syrian activist group Eyes on the City, which monitors Islamic State activity in northern Syria.

According to a Syrian from Raqqa, who asked not to be named for his family's safety, thousands of civilians have fled, most of them to other Islamic State-controlled areas. “Half of the people in Raqqa have escaped, and the other half are afraid,” the man said, citing accounts from one of his relatives in the city.

The U.S. military also believes that the Islamic State has been relocating family members and nonessential personnel out of the city, Warren said.

Most of the areas immediately to the south of the existing front line, at a town called Ain Issa around 30 miles north of Raqqa, are empty desert, and the forces expect to encounter little resistance in the earliest stages of the operation, he said.

Warren declined to say how close to the city the forces intend to move. “We're going to keep the enemy guessing,” he said.

If the advancing forces manage to reach within artillery range of Raqqa, he added, it will further pressure the militants.

The Islamic State has been forced to retreat from numerous locations in recent months and is also currently confronting a major Iraqi offensive to recapture its stronghold of Fallujah in Iraq.