A top US commander has issued a warning saying there are 'tens of thousands' of ISIS fighters spread across Syria and Iraq - despite President Donald Trump's declaration that the terrorist organisation had been defeated.

General Joseph Votel said ISIS fighters are 'dispersed and disaggregated' in the Iraqi-Syria border area, but that there is 'leadership', 'fighters' and 'facilitators' there.

His comments came as the US is set to begin its withdrawal of troops from Syria, following a surprise announcement by Trump in December.

The top US commander in the war against ISIS says there are 'tens of thousands' of fighters spread across Syria and Iraq

'We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,' Trump tweeted in December in announcing his intention to bring back U.S. troops 'NOW.'

The shock tweet helped trigger the resignation of his defense secretary, Jim Mattis, and sent U.S. military officials scrambling to construct a withdrawal plan that preserves as many gains as possible.

Hundreds of additional troops have been sent to Syria to facilitate the withdrawal.

A few weeks after the announcement, four American citizens - two U.S. service members and two civilians - were among 16 people killed in an ISIS terror attack in the U.S.-patrolled town of Manbij.

U.S. officials have long estimated that the Syria pullout could take until sometime in March or April to execute fully, but have been reluctant to set an exact timeline given hard-to-predict battlefield conditions.

His trip comes just as the US plans to withdraw troops out of Syria and will include meetings with foreign leaders to thank them for support

President Donald Trump announced in December that all US troops would be withdrawn from Syria as he claimed ISIS had been 'defeated'

Votel has just embarked on a two week farewell tour in the Middle East to thank foreign leaders for support as he prepares to step down from a 40-year career.

The commander said he is 'very proud' of the force team helping the Syrian Democratic partners in 'difficult fighting', according to CNN.

He said: 'Urban terrain, high density of explosive hazards in the area, and the presence of displaced persons and others who are attempting to depart the area.'

Votel said that all US troops would leave Syria but has been tight-lipped on when it will be completed.

Asked whether the withdrawal of America's more than 2,000 troops would begin in days or weeks, Votel said: 'Probably weeks. But again, it will all be driven by the situation on the ground.'

'In terms of the withdrawal ... I think we're right on track with where we wanted to be,' Votel told reporters traveling with him during a trip to the Middle East.

'Moving people is easier than moving equipment and so what we're trying to do right now is again (to) kind of clear out those materials, that equipment, that we do not need.'

He said: 'The President's orders are very clear to us. We understand exactly what he has directed'.

Meanwhile Syrian fighters backed by artillery fire from a US-led coalition is battling a a fierce jihadist fightback as they pushed to retake a last morsel of territory from ISIS.

Mushrooming black clouds rose over the embattled jihadist holdout in eastern Syria, as missiles and a warplane streaked through the sky.

The Kurdish-led Syrian Democratic Forces (SDF) on Saturday announced the final push to expel hundreds of diehard jihadists from that patch on the Iraq border.

The US-led coalition maintained a steady beat of bombings on the last IS pocket on Monday after an early morning IS counterattack caused several SDF casualties.

'IS launched a counterattack on our forces and we are now responding with rockets, air strikes and direct clashes,' SDF spokesman Mustafa Bali told AFP.