The army "announces a halt in fighting in some areas of Eastern Ghouta in Damascus province from midday today," it said in a statement carried by state news agency SANA.
"The army will retaliate in a suitable manner to any violation" of the ceasefire, the statement said, without specifying what areas were included.
Russia said earlier today it had signed a deal with "moderate" Syrian rebels at peace talks in Cairo on how a safe zone would function in Eastern Ghouta.
But the deal has yet to be fully implemented over disagreements on the monitoring mechanism for the safe zones.
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The most recent talks in Kazakhstan this month between Russia, Turkey and Iran failed to iron out the details of the four safe zones.
Russia said the sides have now signed agreements under which "the borders of the de-escalation zone are defined as well as the deployment locations and powers of the forces monitoring the de-escalation".
Russia said it plans to send in the first humanitarian convoy and evacuate the wounded "in the next few days."
The Eastern Ghouta region, a major rebel stronghold near the capital, has been the frequent target of government military operations.
More than 330,000 people have been killed in Syria since its conflict broke out in March 2011 with anti-government protests.