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Three suspected Islamic State group suicide bombers blew themselves up in northern Iraq on Sunday, the army said, a day after the jihadists' "caliphate" was wiped out in neighbouring Syria.

Army spokesman Yahya Rassoul said the incident took place in a region near the Syrian border, where jihadist sleeper cells are believed to be present.

He said the suspects died as troops surrounded them but there were no casualties among government forces.

Local officials said the suspects were killed as they were trying to attack troops in the village of Qayrawan, south of the mountainous region of Sinjar which borders Syria.

Fighters of the US-backed Syrian Democratic Forces on Saturday pronounced the death of the nearly five-year-old IS "caliphate" which once stretched across a vast swathe of Syria and Iraq.

Their victory was hailed as a major landmark in the battle against the jihadists but there have also been numerous calls for "vigilance" with many saying the fight is far from over.

Top SDF commander Mazloum Kobane on Saturday warned that a new phase had begun in anti-IS operations and appealed for sustained assistance from the US-led coalition to help smash "sleeper cells".

Diehard jihadists continue to have a presence in mountainous or desert regions between Syria and Iraq, which had declared victory over IS in December 2017.

In Iraq some of these regions remain inaccessible to security forces.

Fight against radicalism continues

The bombings came as 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".

"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".