"These de-escalation zones, as I have seen them on the map, are in western Syria and not in areas where ISIS [Islamic State of Iraq and the Levant] is active", he said.
The memorandum provides for halting the hostilities between Syrian government forces and armed opposition groups within the safe zones, allowing humanitarian access, medical assistance, as well as the return of displaced civilians to their homes and the restoration of damaged infrastructure.
That leaves the USA and its allies free to continue the campaign to retake IS-held territory.
He says a cease-fire is unsustainable in the presence of the Iranian-backed militias in Syria, accusing them of also forcefully displacing people to replace them with a loyal population.
Wide parts of Syria have enjoyed relative calm despite sporadic clashes after a deal to set up "de-escalation zones" mostly within opposition-controlled areas went into effect, opposition activists and government media outlets said.
If it worked, the ceasefire could be extended with the agreement of all parties, a memorandum said on Saturday.
In Washington, a Pentagon spokesman said in a statement that Dunford and Gerasimov "talked about the recent Astana agreement and affirmed their commitment to de-conflicting operations in Syria".
Russian Federation said it could be expanded to create more zones.
"As officials from the three countries that back rival sides in the conflict signed the agreement at talks in the Kazakh capital of Astana on Thursday, some members of the Syrian opposition delegation shouted in protest and walked out of the conference room", CBS News and the Associated Press report.
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The top USA military officer General Joseph Dunford and Russia's military chief of staff affirmed their commitment to avoid clashes in the crowded airspace over Syria, the Pentagon said Saturday. U.N. Secretary-General Antonio Guterres said he was encouraged by the agreement.
Osama Abo Zayd, a spokesman for the Syrian military factions at the Kazakhstan talks, said it was "incomprehensible" for Iran to act as a guarantor of the deal.
Along with Mr. Trump, the presidents of Russian Federation and Turkey have recently supported the idea of creating safe zones in Syria.
United Nations envoy Staffan de Mistura, who was in Astana as an observer, described the agreement as "an important promising positive step in the right direction" toward a halt in the fighting.
Russian Federation announced that the safe zones, which went into effect overnight, are now officially closed to all warplanes, and USA officials are livid at the idea, insisting they have no intention of respecting that.
"While we express our full commitment to the political process to end the Syrian crisis, we absolutely reject any role played by the Iranians, who are part of the problem and not the solution".
According to the agreement, the government and opposition forces will halt fighting for six months in four zones, in the northwestern province of Idlib part of Aleppo countryside, the central province of Homs, the Eastern Ghouta countryside of Damascus, and areas in the Daraa and Qunaitera provinces in southern Syria.
The plan is the latest global attempt to reduce violence in war-ravaged Syria.
The new proposal is significantly more ambitious, proposing the deployment of monitoring forces from the guarantor countries and seeking to ground all warplanes.