Fighters of the US-backed Syrian Democratic Forces' victory over the remote riverside village of Baghouz where IS made its last stand capped a deadly six-month operation against the final remnants of IS-controlled territory once stretched across a vast swathe of Iraq and Syria with seven million people in its sway.

"We will remain vigilant... until it is finally defeated wherever it operates," Trump said in a statement.

"The United States will defend American interests whenever and wherever necessary. We will continue to work with our partners and allies to totally crush radical Islamic terrorists."

Trump also had a warning for youths who can easily be swayed.

"To all of the young people on the internet believing in ISIS's Propaganda, you will be dead if you join. Think instead about having a great life," he said.

Acting Defense Secretary Patrick Shanhan welcomed the "critical milestone," but warned that "our work is far from complete."

"We will continue our work with the global coalition to deny ISIS safe haven anywhere in the world," he said.

"We remain committed to ISIS's enduring defeat and we are confident that we will prevail."

Chairman of the Joint Chiefs of Staff General Joseph Dunford, Jr added that "the US military remains committed to working closely with our Coalition and regional partners to ensure the lasting defeat of ISIS."