US Secretary of State John Kerry announced early on Saturday that Syria's new nationwide cease-fire will become effective at sundown Monday with the start of the Qurban Bayram holiday, also known as Eid-al Adha.
If the cessation of hostilities lasts one week, Russia and the U.S. will begin a joint military effort to fight Daesh and al-Nusra in Syria, he added.
The United States and Russia on Friday agreed a plan to impose a ceasefire in the Syrian civil war and lay the foundation of a peace process.
Standing by Russia's Foreign Minister Sergei Lavrov after a day of marathon talks in Geneva, Kerry said he believed the plan would lead to talks to "stop the conflict".
"Today, the United States and Russia are announcing a plan which we hope will reduce violence, ease suffering and resume movement towards a negotiated peace and a political transition in Syria," Kerry added.
"The United States is going the extra mile here because we believe Russia and my colleague have the capability to press the Assad regime to stop this conflict and come to the table and make peace," he said.