U.S. Vice President Mike Pence claimed Wednesday the Daesh terrorist group has been finally defeated just after it claimed an attack that took the lives of a yet determined number of service members and civilians in Manbij, Syria.

"The caliphate has crumbled and ISIS has been defeated," Pence declared at a State Department gathering of more than 180 ambassadors, using another name for Daesh.

The U.S.-led anti-Daesh coalition said the number of American soldiers who were "killed during an explosion while conducting a routine patrol" remains unknown.

Initial reports suggested the attack, reported to be a suicide bombing, claimed the lives of four American service members.

President Donald Trump abruptly announced in December that all U.S. troops would be exiting Syria.

The pullout decision came during a phone call with Turkish President Recep Tayyip Erdogan in which the two leaders agreed on the need for more effective coordination over the civil war-ravished country.

But it has resulted in significant pushback within his administration and on Capitol Hill. Secretary of Defense James Mattis resigned his post shortly after the announcement was made public, and Trump's closest legislative allies have warned against following through with the policy.

Speaking during a committee hearing, Senator Lindsey Graham reiterated his long-standing concerns adding "you had set in motion enthusiasm by the enemy we are fighting.

"I would hope the president would look long and hard of where he is headed in Syria," said Graham.

Trump "has been fully briefed and we will continue to monitor the ongoing situation in Syria," White House spokeswoman Sarah Sanders said in a statement.