Under the memorandum they signed as guarantors of a cease-fire, fighting between government forces and insurgents is to stop in four zones that include rebel-held territory in the north, central and southern parts of the country.
It remains unclear precisely how the guarantors will monitor compliance with what they are calling “de-escalation zones.”
Aleksandr Lavrentyev, the Russian negotiator at the Astana talks, was quoted by Russian news media as saying Russia was prepared to send observers to these zones and “work more closely” with other countries that back the rebels, including the United States and Saudi Arabia.
There was no immediate comment from the United States, which sent an emissary to the Astana talks after a telephone conversation on Tuesday between President Trump and President Vladimir V. Putin of Russia, who vowed to renew efforts to collaborate on ending the Syria conflict.
The zones exclude any areas held by the Islamic State and a Qaeda affiliate commonly known as the Nusra Front, extremist groups that are not participating in the talks and that have been targeted in aerial assaults by forces of Russia and the United States.
Osama Abu Zeid, a spokesman for some of the rebel groups at the Astana talks, said in a statement that they had rejected the memorandum partly because creation of the zones implied a fragmentation of the country and the pact contained no guarantee of “the unity of the Syrian territory.”
Mr. Zeid also said the groups he represented opposed any role as a cease-fire guarantor for Iran and the pro-Assad militias it supports because “they are aggressors.”
Mr. Lavrentyev said at a news conference in Astana that the agreement would go into effect on Saturday and that the Syrian Air Force was expected to avoid the protected zones.
He also said the Syrian government would abide by the agreement, unless rebel groups carried out attacks in the zones — ambiguous language that critics called a loophole that allows violations committed by Mr. Assad’s side.
Many of Mr. Assad’s opponents in the conflict say the Syrian government has no credibility to honor any cease-fire agreement because it has violated all previous such pacts since the conflict began in March 2011.
