Meanwhile, the Ahrar al Sham opposition group enters into a ceasefire agreement with the Tahrir al Sham alliance, under which the former and other opposition factions would "leave the Bab al Hawa crossing and turn it over to civilian administration."
Russia's defence ministry said Saturday that its officials had signed a deal with the Syrian opposition at peace talks in Cairo on how a de-escalation zone near Damascus will function.
The news comes two days after the US announced it would halt its years-long programme to arm and train moderate elements within the Syrian opposition groups battling the Assad regime. Russia had long pushed the United States to end the programme.
"As a result of talks held in Cairo between Russian defence ministry officials and moderate Syrian opposition brokered by the Egyptian side... agreements have been signed on how the Eastern Ghouta de-escalation zone will function," the Russian defence ministry said in a statement.
The regime military declared a cessation of hostilities in Eastern Ghouta region, regime-run television reported.
It said the cessation began at 12:00 noon (0900 GMT) and any violation would receive an "appropriate response".
"Opposition stronghold" Eastern Ghouta
The opposition stronghold of Eastern Ghouta is in one of four proposed "de-escalation zones" designated in an agreement reached by regime allies Iran and Russia and rebel backer Turkey in May.
But the deal has yet to be fully implemented as disagreements persist on the monitoring mechanism for the de-escalation zones.
The most recent talks in Kazakhstan this month between Russia, Turkey and Iran failed to iron out of the details of the four "de-escalation zones."
Russia said that 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."
It said the sides had also agreed "routes to supply humanitarian aid to the population and for free movement of residents."
Russia said it plans to send in the first humanitarian convoy and evacuate the wounded "in the next few days."
The Eastern Ghouta region is a major opposotion stronghold near the capital, and it has been the frequent target of the Assad regime's military operations.
Free Syrian Army under increasing pressure
Mainstream foreign-backed opposition groups fighting under the Free Syrian Army (FSA) banner have increasingly lost out to regime and Russian-backed groups and other rebels.
The US administration's decision to halt a CIA programme to equip and train the group came at a critical time in the Syrian conflict that has split the country into several zones since 2011, when the conflict erupted killing around 400,000 people, displacing millions and giving rise to the terror group Daesh.
A ceasefire on Friday between Hayat Tahrir al Sham, an alliance led by al Qaeda's former Syria branch, and the more moderate Ahrar al Sham plus mainstream opposition factions struck a further blow to the group.
Ahrar al Sham, which had sided with FSA groups fighting against the Tahrir al Sham alliance, said in an online statement that under the ceasefire, it and other opposition factions would "leave the Bab al Hawa crossing and turn it over to civilian administration."
Tahrir al Sham had on Friday advanced in several areas, including towards Bab al Hawa, an important supply route for Turkish-backed opposition groups fighting under the FSA banner.
Tahrir al Sham later released a statement urging Ahrar al Sham fighters at Bab al-Hawa to surrender, saying they would not be harmed if they did.
"To our brothers in Ahrar al Sham leadership, come out strong and dignified - we will give you security and peace and sit together to come up with a project that unifies us," it said.
Opposition fighters say the infighting takes the focus away from the battle against Assad's forces and weakens those who are against the regime.