(CNN) On the same day that an ISIS-claimed attack killed US service members in Syria, Vice President Mike Pence declared that "the caliphate has crumbled and ISIS has been defeated."

Pence's remark to the Global Chiefs of Mission conference at the US State Department came about an hour after the US-led coalition confirmed that American troops had been killed in an explosion in Manbij.

"U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time," the tweet from Operation Inherent Resolve said.

US Central Command announced Wednesday afternoon that "two U.S. servicemembers, one Department of Defense (DoD) civilian and one contractor supporting DoD were killed." The Pentagon confirmed that all four of the victims were Americans. Three additional service members were injured.

"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," the CENTCOM statement said.

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