France's defence minister says Islamic State militants have been weakened but not wiped from the map in Syria and that the fight to defeat them definitively in their remaining pockets needed to carry on.

"Islamic State has been weakened more than ever," Florence Parly said on Twitter on Thursday, responding to President Donald Trump's announcement on Wednesday that the group had been defeated and a US withdrawal would begin.

"But Islamic State has not been wiped from the map nor has its roots. It is necessary that the last pockets of this terrorist organisation be definitively defeated militarily," Parly said.

PA

France, which is a is a leading member of the US-led coalition fighting militants in Syria and Iraq and has, will keep its 1000 odd troops in northern Syria for now.

The French commitment includes special forces units, deployed alongside local Kurdish and Arab forces.

US President Donald Trump declared so-called Islamic State as defeated on Wednesday, saying American ground troops were pulling out of the war-ravaged nation.

"We've won against ISIS," Trump said in a short video posted on Twitter.

"We've beaten them and we've beaten them badly. We've taken back the land. And now it's time for our troops to come back home."

A withdrawal could have extraordinary geopolitical ramifications, and plunges into uncertainty the fate of US-backed Kurdish fighters who have been tackling Islamic State jihadists, thousands of whom are thought to remain in Syria.

AAP

The US has, about 2,000 US forces are in the country, most of them on a cadre mission to support local forces fighting IS.

French diplomats told Reuters on Mr Trump's decision to withdraw all its 2,000 troops from the region had taken Paris by surprise.

"It shows that we can have different priorities and that we must count on ourselves first," Europe Minister Nathalie Loiseau told C-News television.

"For now, of course we are staying in Syria because the fight against Islamic State is essential."

President Emmanuel Macron spoke with Mr Trump on Wednesday, diplomats said.

AAP

In April, when the US leader previously announced a withdrawal, MR Macron persuaded the him that Washington should stay engaged by citing the threat of Iran in the region.

France is especially sensitive to the so-calked Islamic State threat after several major attacks on its soil in recent years.

Hundreds of French nationals have also joined the group in Syria.

French officials are scrambling to find out from their US counterparts exactly what Mr Trump's announcement means for the ongoing conflict.

READ MORE Trump declares victory over IS in Syria, orders US troops home

"We're used to it now with the Trump administration. The devil is in the detail," one French diplomat said.

Australia is also part of the coalition operating in the region and a Middle East export told SBS News the decision to begin removing troops from Syria could have ramifications for Australian forces in the region.

"If IS gains ground, and if the Iranians and the Russians and Turks gain ground in north-eastern Syria, it's going to impact on decisions we have to make down the track," Greg Barton, Chair In Global Islamic Politics at Melbourne's Deakin University, said.

With Reuters