Mr Trump also said that Russia, Iran and Syria had been the biggest beneficiaries of United States action in Syria and it was time to bring American soldiers home.
His top diplomat Mike Pompeo is on a whirlwind regional tour aimed at reassuring allies amid rising tensions between the usa and Turkey over the fate of Washington's Syrian Kurdish allies in the fight against the Islamic State (IS) group.
The US-led coalition on Saturday fired more than 20 missiles against jihadist positions, the observatory said. Turkey considers the YPG to be affiliated with the PKK, which is banned by Ankara as a terrorist organisation.
Trump's announced pullout from Syria without guaranteeing safety for the Kurds, who have helped U.S. forces fight the Islamic State group, had received sharp criticism from politicians.
Ankara sees the backbone of that alliance, the YPG, as a terrorist group linked to the Kurdistan Worker's Party (PKK), which has fought a decades-long insurgency against the Turkish state.
On December 19, White House Press Secretary Sarah Sanders announced that the U.S. started to pull out its troops from Syria.
Colonel Sean Ryan, the coalition spokesman, said: "The SDF is making great progress and continues to liberate more territory once held by ISIS (Islamic State), but the fight continues".
Ahead of Trump's move, Turkish President Recep Tayyip Erdogan threatened to launch another operation in Syria targeting the US-backed Kurdish People's Protection Units (YPG).
The president defended his decision to withdraw troops, saying any remaining IS fighters could be attacked from an unspecified "existing nearby base".
He said the US envoy for Syria, Jim Jeffrey, had traveled to northeast Syria this week and would soon go to Ankara for talks, including on moving forward a UN-led political process to end the eight-year long conflict in Syria.
"We also know that those fighting alongside us for all this time deserve to be protected as well", he added.
After media reports suggesting the departure of United States forces had begun, the Pentagon later said no troops had yet withdrawn and stressed that the battle against ISIS was continuing as US-backed forces try to capture the group's last remaining pockets of territory in Syria.
The previous week, Trump's National Security Advisor John Bolton had announced that Washington would withdraw its troops from Syria only after defeating ISIS and ensuring that the Kurds are safe.
Erdogan rebuffed a proposed meeting with Bolton in Ankara on January 8, then took to live television instead to insult him for a lack of perspective.