With many of its towns and villages ravaged by bombardments in the six-year conflict, Eastern Ghouta near the capital is one of the last bastions of rebels fighting President Bashar al-Assad's regime.
More than 330,000 people have been killed in Syria since its conflict broke out in March 2011 with anti-government protests.
Eastern Ghouta is in one of four proposed "de-escalation zones" designated in a deal reached by government allies Iran and Russia and rebel backer Turkey in May.
The army "announces a halt in fighting in some areas of Eastern Ghouta in Damascus province from midday on Saturday (0900 GMT)," 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 warned, without specifying what areas were included.
Russia said hours earlier it had signed a deal with "moderate" Syrian rebels at peace talks in Cairo on how a safe zone would function in Eastern Ghouta.
The Cairo meeting "follows on from the ceasefire deal for the south of Syria" that took hold on July 9, said Wael Alwan, a spokesman for Faylaq al-Rahman.
That ceasefire for southern areas of Syria was brokered by Russia, the United States and Jordan.
Russia said it and the rebels had 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".
It said the sides had also agreed "routes to supply humanitarian aid to the population and for free movement of residents".
The two other "de-escalation zones" included in the May deal are the rebel-held province of Idlib and northern parts of the central province of Homs.
More than 2.5 million people are believed to live in the four zones.
The May accord roughly laid out the areas where rebels and government forces should halt hostilities, including air strikes, for six months.
However, Russia, Turkey and Iran failed to meet a June 4 deadline to set exact boundaries for the zones.
One major stumbling block appeared to be who would ensure security in all four areas, with Turkey and Iran in particular reportedly wrangling to bolster their influence.
Moscow has argued the zones agreement would provide moderate rebels with security and help focus attacks against jihadist groups such as former Al-Qaeda affiliate Fateh al- Sham Front and the Islamic State group.
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