(CNN) - The initial US assessment of the deadly bombing in Syria that killed four Americans is that ISIS was behind the attack, two US officials said Thursday.
One official said it is believed that Wednesday's attack in Manbij was carried out by an ISIS sleeper cell.
ISIS claimed responsibility for the bombing on Wednesday. The ISIS-affiliated Amaq agency said the attack was carried out by a suicide bomber with an explosive vest.
The American deaths included two US service members, a defense contractor and a Department of Defense civilian, the US Central Command said in a statement. Three other US service members were injured in the attack.
Prior to Wednesday's attack, only two US service members had been killed in action in Syria since the start of the campaign in 2014.
Eight civilians and two Syrian Democratic Forces fighters were killed alongside the four Americans -- a total of 14 people, a senior commander at the Manbij Military Council told CNN Thursday.
The restaurant that was bombed "was frequented by Coalition Forces," the commander said.
"Such attacks were expected. We expected an increase in the attacks, especially that ISIS is in its final stages, but all the attacks that have happened so far have been small," he added.
The attack came less than a month after President Donald Trump announced the withdrawal of US troops from Syria on the premise that the US had "won against ISIS."
Trump offered condolences to the four Americans killed in a speech at the Pentagon Thursday, the first time he had publicly acknowledged the attack over 24 hours after it took place, but quickly changed the subject to border security.
"I want to take a moment to express my deepest condolences to the families of the brave American heroes who laid down their lives yesterday in selfless service to our nation. These are great people, great great people. We will never forget their noble and immortal sacrifice," Trump said.
Trump then said he would "briefly" address the "critical national security" issue at the border, but spoke about the matter at length in front of members of the military at the Pentagon.
The US has about 2,000 troops in Syria, with no specific date for their withdrawal. Last week, the US began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation.
Following Wednesday's attack, two US officials told CNN that there are no current plans to reverse Trump's decision to begin withdrawing US troops from Syria.
The President continues to believe that it's time for US troops to return home, the officials said. Specific withdrawal plans remain contingent on events on the ground, including the strength of ISIS but also security guarantees for the Kurds.
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