WASHINGTON (AFP) - When President Donald Trump vowed US forces would shortly quit Syria, observers were left wondering if he was signalling an epic policy shift - or whether it was more a case of wishful thinking.

Mr Trump, who campaigned on a pledge of "America First", is said to be frustrated by the open-ended US commitment in Syria, where special operations forces are training and advising Kurdish and Syrian Arab fighters who are battling the Islamic State in Iraq and Syria (ISIS) group.

Now that the Islamists have lost 98 per cent of the ground they once held, according to the Pentagon, Mr Trump last week told supporters it is almost time to bring the 2,000 or so US troops home from Syria.

"We'll be coming out of Syria, like, very soon. Let the other people take care of it now," Mr Trump promised in an address to industrial workers in Ohio.

"We're going to have 100 per cent of the caliphate, as they call it - sometimes referred to as 'land' - taking it all back quickly, quickly," he added.

Analysts were quick to question the consequences of a rapid withdrawal from Syria, and by Monday (April 2), it was clear that the current Syria policy is not shifting, at least for now.

"Our mission has not changed," Pentagon spokesman Major Adrian Rankine-Galloway said.

"We are continuing to implement the President's strategy to defeat ISIS."

Officials told AFP that Mr Trump's aside in his speech was not a slip, but that for several weeks he had been resisting the idea of a long- or medium-term US commitment to stabilising eastern Syria after the defeat of ISIS.

The State Department refused to confirm or deny a report in The Wall Street Journal that Mr Trump had placed on hold US$200 million (S$262.33 million) in US funding to help stabilise areas of eastern Syria recaptured from ISIS.

But, speaking on condition of anonymity, US officials confirmed that Mr Trump has been privately questioning the strategy for several weeks.

Since late 2014, the United States has led an international coalition fighting the ISIS group in Syria and Iraq, where most of the combat has been conducted by local partners on the ground.

Mr Trump's apparent readiness to quit the chaos of Syria runs counter to a new US strategy announced in January by then secretary of state Rex Tillerson - who has since been sacked.

Mr Tillerson argued US forces must remain engaged in Syria to prevent ISIS and Al-Qaeda from returning and to deny Iran a chance "to further strengthen its position in Syria".

Mr Charles Lister, a senior fellow and director of the Extremism and Counter-terrorism Programme at the Middle East Institute, noted that a sudden withdrawal of US forces would immediately bolster Iran, Russia and the regime of Syrian President Bashar al-Assad.

Mr Trump's "comments come so much out of nowhere that it's hard to take them totally seriously", said Mr Lister, of the Washington think tank.

The President has picked a new national security adviser, Mr John Bolton, and nominated a new secretary of state, Mr Mike Pompeo, both of whom are hawkish on Iran.

"Trump and his new team basically now are kind of violently anti-Iranian and leaving Syria would represent a total US capitulation to Iranian influence in the region," Mr Lister told AFP.

Mr Trump's advisers including Defence Secretary James Mattis and other officials "will at least for now overpower Trump's tendency to just pack up and leave".

But Mr Daniel Davis, a retired army lieutenant colonel and fellow at the Defence Priorities military think tank, said Mr Trump is right to desire a withdrawal of US troops because ultimately, America's presence would have no impact on Syria's seven-year civil war, and that the real players are Iran and Russia.

"Why would the President want to continue to do something that is guaranteed to fail? I think (withdrawal) is the right thing to do," Mr Davis told AFP, noting that America should only get involved in foreign military interventions when US national security interests are directly threatened.

More than 340,000 people have been killed and millions displaced since Syria's civil war started in 2011, spiralling into a complex conflict involving multiple world powers.