An Islamic State backer in the streets of Raqqa, Syria, in 2014. (Reuters)

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

Although the operation appears to have relatively limited goals, it will serve as an early test of a coalition being forged with U.S. help between local Arab fighters and the Kurdish People’s Protection Units, or YPG, to take on the militant group in its most symbolically significant stronghold.

A few thousand Kurdish and Arab fighters — grouped under the umbrella of the U.S.-backed Syrian Democratic Forces (SDF) and backed by U.S.-led coalition airstrikes — began moving south 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 in the mostly desert terrain north of Raqqa, said Col. Steve Warren, a spokesman for the U.S. military, speaking by telephone from Baghdad.

He described the operation as a significant step toward an eventual bid to recapture Raqqa and said it would add to the pressures confronting the Islamic State in its territories 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 form of offensive was planned for Raqqa. It was the first major city captured by the Islamic State when it first began conquering territory in 2013, and Raqqa has since served as a hub for the militant group’s operations — the first destination for foreign fighters and the focus of its propaganda.

Warplanes with the U.S.-led coalition dropped leaflets over the city late last week urging civilians to flee. On Friday, the Islamic State lifted a months-long ban on all travel out of Raqqa, triggering an exodus of thousands of people, according to residents.

A Syrian man from Raqqa, who spoke on the condition of anonymity for fear of his family’s safety and now lives in Turkey, said relatives told him that everyone they knew was trying to get out. “Half of the people in Raqqa have escaped, and the other half are afraid,” the man said.

A statement by the Islamic State’s chief spokesman on Saturday apparently suggesting that the group is at risk of losing control of Raqqa heightened the sense of unease, said Ahmed Mhidi of the Syrian activist group Eyes on the City, which monitors Islamic State activity in northern and eastern Syria.

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

The offensive comes as the Islamic State is on the retreat from numerous front elsewhere in Iraq and Syria. It is battling to counter a major Iraqi offensive launched Sunday to recapture the militant group’s stronghold of Fallujah in Iraq.

The campaign on the Raqqa front had long been stalled by political quarrels between Arabs and Kurds in the area, which is under the control of the Kurdish YPG but inhabited mostly by Arabs. The SDF was formed in October, under U.S. auspices, in an attempt to bring Arabs and Kurds together in the fight against the Islamic State.

The SDF has claimed some important victories elsewhere in northern Syria, but the force remains overwhelmingly dominated by the Kurds, and efforts to recruit Arab fighters have been plagued by disputes and mistrust.

The U.S. military in April dispatched 250 Special Operations forces to northern Syria to join the 50 who were already there, with the goal of helping recruit and train more Arab fighters north of Raqqa.

The YPG has expressed little interest in fighting for Raqqa, which is a predominantly Arab town, and U.S. officials say a significantly larger Arab force is needed before launching an assault directly on the city. But U.S. officials have not put a time frame on how long it could take to assemble enough Arab fighters to retake the key city.