“The caliphate has crumbled and ISIS has been defeated,” said Vice President Mike Pence just hours after the deadly attack.
A Turkish push in northern Syria carries a risk of U.S. forces getting caught in the crossfire.
Some Syrians in the town celebrated by cutting off Islamic State-required beards and burning burqas, while others took part in previously prohibited activities like smoking and dancing.
Video footage from the city of Manbij in northern Syria shows a crowd cheering as a woman waves a swath of black fabric and sets it on fire.
The Islamic State has ignored an offer for its fighters to withdraw safely from the Syrian city of Manbij, and clashes with US-backed forces continued after the offer expired.
Human rights groups say US-led coalition airstrikes killed at least 56 civilians in and around the besieged city of Manbij in northern Syria on Monday and Tuesday.
US-backed forces fighting Islamic State near the Syrian-Turkish border said on Thursday they had reached the militants' last main route in and out of their stronghold in the area, the city of Manbij.