Fight hardly over

Shortly after news of McGurk's resignation broke, Trump again defended his decision to pull all of the roughly 2,000 US forces from Syria in the coming weeks.

"We were originally going to be there for three months, and that was seven years ago-we never left," Trump tweeted. "When I became President, ISIS was going wild. Now ISIS is largely defeated and other local countries, including Turkey, should be able to easily take care of whatever remains. We're coming home!"

Although the civil war in Syria has gone on since 2011, the US did not begin launching airstrikes against the IS group until September 2014, and US troops did not go into Syria until 2015.

The US allied forces have won significant victories against the IS group in northern Syria, retaking key cities such as Raqqa. But McGurk, along with other US government agencies, believed the fight against the militant group was hardly over.

"It would be reckless if we were just to say, well, the physical caliphate is defeated, so we can just leave now," McGurk told reporters in a Dec 11 briefing at the State Department. "Nobody is declaring a mission accomplished."

By contrast, Trump has claimed victory against Islamic State and considers the mission in Syria over given the group's territorial losses. "Now ISIS is largely defeated and other local countries, including Turkey, should be able to easily take care of whatever remains," Trump wrote on Twitter on Saturday.

Trump's declaration of triumph has alarmed key NATO allies such as France and Germany, who said the change of course on Syria risks damaging the fight against IS group, which has now been squeezed to a sliver of Syrian territory.

Mattis, perhaps the most respected foreign policy official in the administration, announced on Thursday that he will leave by the end of February. He told Trump in a letter that he was departing because "you have a right to have a Secretary of Defense whose views are better aligned with yours".

The IS group still hold a string of villages and towns along the Euphrates River in eastern Syria, where they have resisted weeks of attacks by the US-supported Syrian Democratic Forces to drive them out. The pocket is home to about 15,000 people, among them 2,000 IS fighters, according to US military estimates.

But that figure could be as high as 8,000 militants, if fighters hiding out in the deserts south of the Euphrates River are also counted, according to according to the Syrian Observatory for Human Rights, which monitors the conflict through networks of local informants.

AP - REUTERS