In a joint statement, Washington and Moscow said the partial truce would begin at midnight Damascus time (2200 GMT Friday) and apply to parties to the conflict that have committed to the deal -- but not to the Islamic State (IS) group or Al-Nusra Front, an Al-Qaeda affiliate.
Parties wishing to be included in the agreement have until noon Damascus time on Friday to inform Moscow or Washington of their intention to honour the ceasefire.
"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," US Secretary of State John Kerry said.
US President Barack Obama and Russia's President Vladimir Putin discussed the deal in a phone call, the White House said.
"This is a moment of opportunity and we are hopeful that all the parties will capitalise on it," White House press secretary Josh Earnest said.
UN Secretary General Ban Ki-moon called the ceasefire a "long-awaited signal of hope", and urged all sides to abide by it.
Putin said Moscow will do "whatever is necessary" to ensure that Damascus respects the agreement.
"We are counting on the United States to do the same with its allies and the groups that it supports," he said in a televised address.
- Opposition commitment conditional -
There was no immediate reaction from Damascus, but the main grouping of opposition factions said it "agreed to respond positively to international efforts to reach a truce deal".
President Bashar al-Assad issued a decree for parliamentary elections on April 13 shortly after the truce was announced.
The opposition High Negotiations Committee said its "commitment to the truce is conditional" on the lifting of sieges, release of prisoners, a halt to bombardment of civilians and the delivery of humanitarian aid.
On Monday, state news agency SANA reported a convoy of 44 trucks carrying humanitarian aid had entered Moadamiyet al-Sham, near Damascus, overseen by the Red Crescent and the United Nations.
Once the cessation of hostilities takes hold, the United Nations will work to secure "access to as many places as possible in order to deliver humanitarian aid," UN spokesman Stephane Dujarric said earlier.
The 17-nation group backing Syria's peace process agreed at a meeting in Munich to implement a ceasefire within a week, but the truce never materialised.
Ban said the truce announced Monday "contributes to creating an environment conducive for the resumption of political negotiations", which had been scheduled to resume this week.
A halt in hostilities in Syria would come after five years of brutal civil war that have killed more than 260,000 people and seen half the population displaced, including over four million overseas.
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 IS jihadists.
- Deadliest jihadist attack -
IS on Sunday claimed responsibility for two deadly attacks in regime-held areas, which a monitor said killed 134 people near the Sayyida Zeinab shrine south of Damascus and at least 64 in the Al-Zahraa district of Homs.
The bombings near the shrine marked the deadliest jihadist attack since Syria's conflict erupted in March 2011, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
Russia began air strikes in Syria last September against what it said were "terrorists", but has been accused of bombing non-jihadist rebel forces in support of Assad, a longtime ally.
Iran has sent military advisers to Syria and the Tehran-backed Lebanese Shiite movement Hezbollah has deployed at least 6,000 militants to fight alongside Assad's forces.
Tehran would have to be on board for any ceasefire to work, and Russian Defence Minister Sergei Shoigu made a surprise visit to Tehran on Sunday, saying he was delivering a "special message" from Putin to President Hassan Rouhani.
The rise of IS, which has seized large parts of Syria and neighbouring Iraq and become the preeminent global jihadist group, has focused attention on the need for a solution.
The group has used its ultra-radical view of Islam to justify kidnapping minorities, including Assyrian Christians in northeast Syria.
On Monday, IS released 42 Assyrians, the last remaining hostages from a wide-scale kidnapping in the northeast province of Hasakeh nearly one year ago, the Assyrian Monitor for Human Rights said.
But IS also advanced against government forces in northern Aleppo province, cutting the only supply route linking the west of Aleppo with other government-held territory, the Observatory said.
Fierce clashes were raging in the area, and if government forces are unable to recapture the road, it could slow their offensive.
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