Islamic State has been defeated in Syria, U.S. Vice President Mike Pence said on Wednesday, hours after Americans were killed in a northern Syria bomb attack claimed by the militant group.

Pence did not mention the attack in his address to 184 chiefs of U.S. diplomatic missions who gather annually in Washington from around the world to discuss foreign policy strategy.

"The caliphate has crumbled and ISIS has been defeated," Pence told the U.S. ambassadors and other senior American diplomats, referring to Islamic State.