President Obama has said the coming weeks will be "critical" for Syria's future, ahead of a two-week truce due to start on Friday.
He warned Russia that all parties to the cessation of hostilities needed to stop attacks, including air strikes.
If successful, he said, the truce could be a first step towards ending the chaos and violence in Syria.
And he vowed to defeat the so-called Islamic State (IS), which he said was "not a caliphate but a crime ring".
Mr Obama said the success of the cessation of hostilities would depend on whether parties including the Syrian government, Russia and their allies lived up to their commitments.
Attacks needed to end and humanitarian aid had to be allowed through to desperate civilians, he said.
"The coming days will be critical and the world will be watching," he said.
Earlier, Russian Foreign Minister Sergey Lavrov said he hoped the US would respect the truce.