President Biden announced on Sunday that United States military forces have been conducting airstrikes in Syria to keep the Islamic State from reasserting itself in the aftermath of the fall of President Bashar al-Assad’s government.

Speaking at the White House, Mr. Biden celebrated Mr. Assad’s fall as a “fundamental act of justice” and expressed hope that after decades of repressive rule, the people of Syria could build a new, freer society. But he warned that it was also “a moment of risk and uncertainty” and that his administration would strive in its final days to prevent terrorists from regaining traction.

“We’re cleareyed about the fact that ISIS will try to take advantage of any vacuum to reestablish its capability, to create a safe haven,” Mr. Biden said in his televised remarks from the Roosevelt Room, using another term for the Islamic State. “We will not let that happen.”

“Just today,” he added, “U.S. forces conducted a dozen precision strikes, airstrikes, within Syria targeting ISIS camps and ISIS operatives.”

Mr. Biden did not elaborate on the military actions, but U.S. officials planned to brief reporters later in the day.

In his remarks, Mr. Biden mentioned Austin Tice, the American journalist who has been held in Syria for a dozen years. Asked about his fate, Mr. Biden paused as he was walking out to say: “We believe he’s alive. We think we can get him back.”