"At noon local time tomorrow, a de-escalation zone in southwest Syria will begin to take effect," HR McMaster said in a statement released Saturday.
"Such zones are a priority for the United States, and we're encouraged by the progress made to reach this agreement," he said.
"The United States remains committed to defeating ISIS, helping to end the conflict in Syria, reducing suffering, and enabling people to return to their homes. This agreement is an important step toward these common goals," he said.
McMaster, who released his statement shortly after the US president and other world leaders wrapped up the G20 summit in Hamburg, Germany, said Trump "discussed the agreement with many world leaders at the G20 Summit," including Turkey's President Recep Tayyip Erdogan, Britain's Prime Minister Theresa May, and Germany's Chancellor Angela Merkel.
The decision to impose a ceasefire zone, announced Friday at the summit by Moscow's Foreign Minister Sergei Lavrov, was reached with the US, Russia and Jordan.
Lavrov said the ceasefire would be supervised by Russian military police "in coordination with the Jordanians and Americans."
Syria's conflict evolved from a bloody crackdown on protests in 2011 to a devastating war that has drawn in world powers, including Russia and the US-led international coalition.
More than 320,000 people are estimated to have been killed and millions have been displaced.
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