The Syrian government and rebel delegations are not signatories, but Syrian state news agency SANA reported that DamascU.S. supports the Russian plan.
Russia's deputy defense minister said on May 5 that the implementation of the memorandum on de-escalation zones would put an end to the war.
It provides for a ceasefire, rapid deliveries of humanitarian aid and the return of refugees after the creation of "de-escalation zones" across stretches of eight Syrian provinces.
Syrian opposition activists and government media outlets are reporting relative calm in wide parts of Syria after a deal hammered out by Russia, Turkey and Iran to set up "de-escalation zones" in mostly opposition-held area went into effect.
TRT World's Abubakr al Shamahi brings the latest.
While the deal and precise boundaries of the zones are still being developed, it could also be the first to involve monitoring by foreign troops in Syria, which has been embroiled in a bitter civil war for six years.More news: 500M Windows 10 devices, half of 2018 goal
The United States is not party to the agreement and the Syrian rivals have not signed up to the deal. But the armed opposition was highly critical of the proposal, saying it lacked legitimacy.
"The case is that in the de-escalation zones, the work of aviation, especially the coalition forces, is absolutely not envisaged, with or without notification", Lavrentyev said.
Activist Ibrahim al-Idlibi, who is based in Idlib, said the situation was calm in the province, which has been the target of daily raids by regime and Russian planes for the past few months.
In the tangled mess that constitutes Syria's battlefields there is much that can go wrong with the plan, which emerged from a summit in Kazakhstan.
The proposal calls for taking measures to reduce fighting in four designated areas of Syria where rebels not associated with Islamic State terrorists control significant territory.
Alexander Lavrentyev, Russia's envoy to the peace talks in Kazakhstan, said Friday in remarks covered by Russian media that "the operation of aviation in the de-escalation zones, especially of the forces of the worldwide coalition, is absolutely not envisaged, either with notification or without", he said. The third is around Eastern Ghouta, a rebel-held suburb of Damascus that the government has failed for years to take.More news: Europa League: Man U pip Celta Vigo
The Observatory said government forces shelled and bombed the nearby town of Latamneh and surrounding areas.
The agreement aims to establish four "de-escalation zones", where Syrian government and rebel forces are supposed to stop fighting each other. It was not immediately clear if the fighters were members of the al-Qaida-linked Levant Liberation Committee that is usually excluded from cease-fires.
Doyle said there are huge question marks over the commitment of various actors to reducing violence and over how the plan will be enforced. There were no immediate reports of casualties.
There were reports that the safe zones would be no-fly areas for warplanes from the US-led coalition.
Several forces have carried out air strikes within Syria, including Russian Federation and a US-led coalition battling IS.
The Pentagon said the de-escalation agreement would not affect the US-led air campaign against IS.More news: Obama to deliver keynote speech in Italy on food security
"Apart from a few exchanges and bombardments during the night and in the morning in Hama, Damascus and Aleppo provinces, violence was sharply reduced in the areas covered by the deal", Observatory chief Rami Abdel Rahman told AFP.
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