Fighting in the rebel bastion of Eastern Ghouta, near Damascus, and the province of Latakia is set to halt at midnight on Friday (2100 GMT).
Syria's army said the freeze would come into effect at 1:00 am and would last for 24 hours in Eastern Ghouta and for 72 hours in coastal Latakia, the heartland of President Bashar al-Assad's Alawite sect.
There was no mention of Aleppo, where a week of fighting has killed more than 200 civilians.
In February, the US and Russia brokered a partial truce in Syria between regime forces and non-jihadist rebels.
The US special envoy for Syria, Michael Ratney, said Friday that the agreement was a "general recommitment" to that truce, "not a new set of local ceasefires."
"Likewise, persistent violations in Aleppo have stressed the Cessation of Hostilities and are unacceptable," he said.
"We are talking to Russia to urgently agree on steps to reduce violence in that area as well."
A Syrian security source in Damascus said the deal had been reached in Geneva between US and Russian officials.
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"The Americans asked for Aleppo to be included, but the Russians refused," the source said.
Russia is a key backer of Assad's regime, while the US has supported various opposition factions in the country.
A diplomatic source quoted by Russian state news agency RIA Novosti said that Moscow and Washington, co-chairs of the International Syria Support Group, "are the guarantors of the 'regime of silence' implementation by the sides."
Russian Lieutenant-General Sergei Kuralenko, based in the Hmeimim airbase in Latakia province, said that during the freeze, "all combat and using any weapons will be forbidden".
"We call upon all parties interested in establishing peace on Syrian land to support the Russian-American initiative and not disrupt the 'regime of silence'," he said, quoted by RIA Novosti.
Although the February 27 truce had seen violence drop across large parts of the country, fighting against jihadist groups continued in Latakia, the eastern province of Deir Ezzor and elsewhere.
Eastern Ghouta is held by the powerful Jaish al-Islam (Army of Islam) rebel group, which has signed on to the truce.
But fighting there between Jaish al-Islam and regime forces has been building in recent weeks.
Opposition factions in Eastern Ghouta and Latakia were not immediately available for comment on the freeze.
More than 270,000 people have been killed since Syria's conflict erupted in March 2011 with anti-government protests.