BEIRUT — U.S. aircraft ferried Syrian Kurdish fighters and allied forces behind Islamic State lines on Wednesday to spearhead a major ground assault on a strategic town held by the extremist group outside its self-declared capital, Raqqa, the Pentagon said, marking the first time U.S. forces have provided airlift for local forces on a combat operation in Syria.

The airlift was part of what Pentagon spokesman Eric Pahon described as a large, high-priority offensive to secure the area around Tabqa and the associated Tabqa Dam on the Euphrates River which supplies electric power to the area.

“This is a significant strategic target,” said Pahon. If successful, the operation would “basically cut ISIS off” from the western approaches to Raqqa.

The U.S. has significantly widened its footprint in northern Syria as it prepares for the operation to push the militants from Raqqa, deploying a Marine artillery unit and a few dozen Army Rangers in addition to special operation troops and advisers to assist the local forces.

Wednesday’s airlift, which Navy Capt. Jeff Davis, a Pentagon spokesman, described as a first, displayed a new level of commitment to Syria’s Kurds, whose partnership with the U.S. in fighting IS has prompted difficult discussions with Turkey, which sees the militants as a national security threat.

Col. Joseph Scrocca, a spokesman for the U.S.-led coalition that is overseeing the counter-IS campaign in Syria and Iraq, said multiple U.S. aircraft were used to land the Syrian fighters south of Tabqa. The U.S. also provided artillery fire from a Marine contingent, as well as close air support by U.S. Army Apache helicopters, he said.

“This is pretty major,” he said, adding the fight for the dam, the town and the airfield is expected to last at least a couple of weeks.