WASHINGTON, March 22 (Xinhua) -- The Pentagon confirmed on Wednesday the U.S.-led coalition had offered airlift for Syrian rebels fighting the Islamic State (IS) near its de facto capital of Raqqa, Syria.

"This is the first time we have conducted an air movement, air assault, with the Syrian Arab Coalition and Syrian Democratic Forces (SDF)," said a Pentagon official Col. Joseph Scrocca at a Pentagon briefing.

Calling the air operation a "daring assault behind enemy lines," Scrocca said the goal of the airlift was to help Syrian fighters retake a major dam near Raqqa.

Meanwhile, CNN quoted a source as saying that about 500 troops were airdropped behind enemy lines.

According to Syrian Observatory for Human Rights, the airdrop happened after midnight Wednesday in the town of Krein, some five kilometers west of the city of Tabaqa, in the countryside of Raqqa.

The London-based watchdog group said the U.S. ground forces came to help the Kurdish-led SDF fighters to cut the road between Raqqa and the northern province of Aleppo, and to isolate Tabaqa from Raqqa city, thus tighten the noose on the IS militants in the two cities.

The operation, the group added, also aims to prevent the Syrian government forces from closing in on Tabaqa, and the military airport under the same name in that area, in case the later succeeded to capture the town of Maskaneh in the eastern countryside of Aleppo, where the Syrian army is making big progress in battles against the IS group in that part of the country.