IS defeated in Syria, but fight against 'radical Islamic terrorists' will continue: Trump

President Donald Trump welcomed the formal defeat of the Islamic State group in Syria, but said the US will continue to remain "vigilant" against "radical Islamic terrorists".

It follows an announcement by the US-backed Syrian Democratic Forces that the last territories held by IS had been taken.

For weeks, SDF have battled the group at the riverside village of Baghouz, where IS made its last stand, as thousands of civilians and fighters fled the holdout.

Trump welcomed the announcement by the SDF and said that the US will continue to battle IS ad other jihadi groups.

"We will remain vigilant... until it is finally defeated wherever it operates," Trump said of IS 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."

He also warned youth not to fall for the lure of IS in the future.

"To all of the young people on the internet believing in ISIS' Propaganda, you will be dead if you join. Think instead about having a great life," he said, using an alternative acronym for the jihadist movement.

Trump has claimed 16 times that IS had been defeated or on the verge of collapse.

US acting Defence Secretary, Patrick Shanahan, welcomed the "critical milestone", but said 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' enduring defeat and we are confident that we will prevail."

US-led Operation Inherent Resolve Commanding General Paul LaCamera also confirmed the defeat of the group but urged caution in a statement.

"The end of the so-called physical caliphate is a historic military accomplishment that brought together the largest coalition in history, but the fight against Daesh and violent extremism is far from over," he said.

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".

Some IS militants are still present in eastern Syria's Badia desert and hideouts, with the US fearing that the group could launch a guerrilla insurgency.

Analysts have warned remnants of the group have hidden among the local population, and could launch bomb attacks.