The Pentagon Friday confirmed it had begun pulling out some equipment from Syria as part of the pull-out, but that some 2,000 US forces would remain in place until conditions on the ground allowed for them to leave.
The Pentagon released a statement on Twitter that said: 'U.S. service members were killed during an explosion while conducting a routine patrol in Syria today.
The Pentagon said two USA servicemembers, a Department of Defense civilian employee and one contractor working for the military were killed and three servicemembers were injured in the blast in the northern Syria town of Manbij.
The American president did not immediately react to the U.S. troop deaths, but Vice President Mike Pence double-downed on Trump's declaration that the USA would pull all of its 2,000 troops from Syria.
Washington, which has relied heavily on the Kurds in its campaign against IS in Syria, has sought guarantees for their safety after Trump suddenly announced the troop pullout.
USA and allied warplanes and artillery pieces launched 575 strikes against the group in Syria between December 30 and January 12.
White House spokesperson Sarah Sanders said President Trump had been "fully briefed" on the incident. During the summer of 2016, the USA -backed Syrian Democratic Forces (SDF), of which the Kurds make up a significant portion, retook the city to the dismay of Turkey, which didn't like the Kurds operating so far west into Syria and close to their border.
The explosion, reportedly caused by a suicide bomber, hit near the main market in the northern city of Manbij, witnesses told Reuters. "We've taken back the land and now it's time for our troops to come back home".
There were unconfirmed initial reports that the coalition troops killed were Americans.
But he added that the U.S. would "stay in the region and we'll stay in the fight to ensure that ISIS does not rear its ugly head again".
No confirmation of the attack or possible American casualties has been made by USA military sources.
In a speech at the U.S. State Department Wednesday morning, Pence credited President Trump and the U.S. military with vanquishing Islamic State terrorists in Syria. "Will attack [ISIS] again from existing nearby base if it reforms", the president tweeted on Sunday. "Likewise, do not want the Kurds to provoke Turkey".
With the United States extricating itself from Syria, Turkey is poised to move in and fight the jihadists.
On Tuesday, Turkey's President Recep Tayyip Erdogan said Ankara would set up a "security zone" in northern Syria following a suggestion by Trump. And now I'm seeing it in Syria'.
"While the president is promising dialogue with Chairman Kim we still await concrete steps by North Korea to dismantle the nuclear weapons that threaten our people and our allies in the region", he said.