WASHINGTON — Hundreds of Syrian fighters and their American military advisers, backed by American artillery and attack helicopters, have begun a major operation to cut off the western approaches to Raqqa, the city the Islamic State has proclaimed as its capital, officials said Wednesday.

It was the first time that the United States had carried out an air assault with Syrian fighters in its campaign against the Islamic State, and the mission, which began Tuesday, reflected the leeway the Trump administration has given its commanders to carry out operations without prolonged review in Washington.

In a significant commitment of American forces, American helicopters ferried fighters across enemy lines while Marine Corps howitzers, Army Apache attack helicopters and American warplanes provided firepower for the operation. Army surface-to-surface Himars rockets, which are based in northern Syria, are also part of the mission. American Special Operations forces were advising the Syrian fighters on the ground, although a military spokesman said they were not involved in direct, front-line combat.

The mission represented a new stage in the broader offensive to cut off and seize Raqqa. Its objective was to take control of the Tabqa Dam on the Euphrates River near Lake Assad, the nearby town of Tabqa and a local airfield. An immediate goal was to take the dam intact, but the structure was still under the control of the Islamic State, also known as ISIS or ISIL, and officials said that the fighting was intense.