In a joint statement, the two countries said Saturday's partial truce would begin at midnight Damascus time (2200 GMT Friday) and would apply to parties to the conflict that have committed to the deal -- but not to the Islamic State group or Al-Nusra Front, an Al-Qaeda affiliate.
US Secretary of State John Kerry, who has been working with his Russian counterpart Sergei Lavrov to secure an agreement, welcomed the deal.
"If implemented and adhered to, this cessation will not only lead to a decline in violence, but also continue to expand the delivery of urgently needed humanitarian supplies to besieged areas and support a political transition to a government that is responsive to the desires of the Syrian people," Kerry said in a statement.
Parties wishing to be included in the agreement have until noon Damascus time (1000 GMT) Friday to inform Russia or the United States of their intention to honor the ceasefire.
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In return, the groups would be assured of protection from Russian and US-led coalition planes. The two powers are pursuing separate air wars in Syria, with Russia pounding rebel targets and the coalition focused on the IS group.
The Pentagon stressed that any ceasefire would not alter the US-led coalition campaign against the IS group.
"ISIL has not been party to this," Pentagon spokesman Captain Jeff Davis said, using an alternate acronym for the IS group.
"Our operations will continue as they have. The coalition will continue to strike against ISIL.