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(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.

ISIS claimed responsibility for the attack, which the ISIS-affiliated Amaq agency said was carried out by a suicide bomber with an explosive vest.

Pence’s press secretary Alyssa Farah tweeted Wednesday morning that he had been briefed and that he and President Donald Trump were “monitoring the situation.”

The Vice President made no mention of the attack and did not offer condolences in his remarks.

A White House official said the administration had not “publicly confirmed the deaths at the time (Pence) spoke,” even though the coalition against ISIS tweeted a message confirming there had been US deaths nearly an hour before his speech.

Pence released a statement Wednesday afternoon expressing condolences to “the loved ones of the fallen” and condemning the attack. However, the vice president maintained that the ISIS caliphate had been “devastated.”

“Thanks to the courage of our Armed Forces, we have crushed the ISIS caliphate and devastated its capabilities. As we begin to bring our troops home, the American people can be assured, for the sake of our soldiers, their families, and our nation, we will never allow the remnants of ISIS to reestablish their evil and murderous caliphate - not now, not ever,” it said.

Prior to Wednesday’s attack, only two US service members had been killed in action in Syria since the start of the US campaign there in 2014.

The attack comes as the US prepares to withdraw troops from Syria – a decision Pence defended in his Wednesday morning remarks.

“Thanks to the leadership of this commander in chief and the courage and sacrifice of our armed forces, we’re now actually able to begin to hand off the fight against ISIS in Syria to our coalition partners and we’re bringing our troops home,” he said.

However, he said the US would “stay in the region and we’ll stay in the fight to insure that ISIS does not rear its ugly head again.”

“We will protect the gains that our soldiers and our coalition partners have secured,” he said.

CNN’s Zachary Cohen, Veronica Stracqualursi, Kevin Liptak and Jim Acosta contributed to this report.