"We will continue to act against Iran's attempt to establish a military presence in Syria, and if the need arises, we will even expand our activities there".
Justifying his decision on Twitter yesterday, Mr Trump said: "On Syria, we were originally going to be there for three months, and that was seven years ago - we never left".
Many U.S. politicians and worldwide allies fear the withdrawal is premature and would further destabilize the already devastated region.
"Did Erdogan blackmail the president?"
Many analysts fear the move will also allow Iran to more easily spread weapons and fighters throughout the Middle East.
Via tweets on Monday, Trump also lashed out at Brett McGurk, the us envoy for the global coalition to defeat the Islamic State (IS), who resigned on Friday following Mattis' announcement to leave.
Trump stunned his allies and the USA establishment on Wednesday when he ordered the withdrawal of 2,000 American ground forces in Syria.
In a May 2017 Pentagon briefing, Defense Secretary Jim Mattis was flanked by perhaps the two most important USA officials coordinating the fight against the Islamic State.
According to Ravid's report, Israel is "very concerned" the withdrawal will encourage Iran to continue its military buildup in Syria. It did not provide further details.
For its part, The Washington Post signalled that the Trump administration had set new goals for its military contingent in the region and would keep the 2,200 U.S. troops there "indefinitely".
The decision also caused alarm and dismay in the USA military over the prospect of suddenly abandoning Washington's Kurdish partners.
"We literally played back to them that you've got to change your brand, you know, what do you want to call yourself besides the YPG, and with about a day's notice they declared that they were the Syrian Democratic Forces", Thomas told the Aspen Security Forum 2017 (pdf).
For the 17 months, Shanahan was Mattis' No. 2 in the Pentagon.
Still, Trump has long floated the idea of Washington's regional allies filling the void left in the Middle East by the United States as it pulls assistance and scales back its military presence in the region in pursuit of his "America First" policies.
"I am quite sure that, at this stage, President Bashar al-Assad will have no great problem with Trump taking the position that it is up to other nations to foot the bill of rebuilding Syrian infrastructure destroyed by United States allies enabled with U.S. weapons".
A U.S. soldier walks on a newly installed position, near the tense front line between the US-backed Syrian Manbij Military Council and the Turkish-backed fighters, in Manbij, north Syria, April 4, 2018.