The insurgent groups suspended participation in the talks on Wednesday to protest what they described as heavy bombing by the Syrian government’s Russian-backed forces the day before that killed dozens, including civilians.
The Russian proposal does not specify measures to prevent government warplanes from carrying out such bombings. Rebels said they remained suspicious of Russian guarantees, regardless, because Russia has been unable or unwilling to curb government attacks on civilians.
President Vladimir V. Putin of Russia said on Wednesday that the proposal had the backing not only of Russia but also of Iran, another ally of President Bashar al-Assad of Syria, and Turkey, which backs some anti-Assad groups.
“We as guarantors — Turkey, Iran, Russia — will do everything for this to work,” Mr. Putin said in remarks carried on Russian television, speaking in Sochi, Russia, after meeting with President Recep Tayyip Erdogan of Turkey.
The proposal was made as the United States, another supporter of some anti-Assad groups, appeared to be re-engaging in the negotiations after a prolonged absence.
Stuart E. Jones, the acting assistant secretary of state, was in Astana, the most senior American official to participate in Syria talks since President Trump took office.
He arrived after Mr. Trump and Mr. Putin held a phone conversation on Tuesday about renewing efforts to resolve the conflict, which has left hundreds of thousands dead and half the population displaced.
The draft proposal calls for “de-escalation zones” of safety to be established in four areas: Idlib Province, almost entirely held by jihadist and other rebel groups; Eastern Ghouta, a large area of the Damascus suburbs besieged by government forces; a besieged pocket north of the central city of Homs; and southern Syria along the Jordanian border, where rebel groups backed by the United States and its allies have made gains in recent months against both Islamic State and government forces.
Under the proposal, checkpoints ringing those areas would be maintained by both government and rebel forces to allow the free movement of civilians and relief aid. That provision could offer respite from siege warfare, which has been a main weapon of the government.
The proposal also says rebel groups would be required to fight the Islamic State and the formerly Qaeda-affiliated Nusra Front, now called Tahrir al-Sham, which are not part of any peace process.
But the proposal offers few details on how fighting would be thwarted inside the secure zones.
An earlier draft circulated by some opposition members included a provision that Syria’s air force would be grounded in those zones — but no mention is made of that provision in a longer draft. It was removed, participants said, because of Syrian government objections. But without that provision, rebels would probably reject the proposal.
Analysts in Damascus close to the government of Mr. Assad said the government had rejected any proposal that would accept rebel control of any area, even temporarily. The government has long insisted that it aims to take back all of the country, and it has so far refused any territorial or political compromise with its opponents.
The proposal raises the possibility of outside forces helping to guarantee a cease-fire. It says military units or “guarantors” would be deployed as monitors.
Rebel representatives said they would not accept any from Iran or Russia. Russian news outlets, including the news agency Interfax, said the forces could be from former Soviet states — Kazakhstan was floated as a possibility — or members of the block of emerging economies that include Russia, Brazil and India. Those reports also mentioned Arab countries, leading to speculation that Egypt could contribute. Egyptian officials have denied any intention of sending forces to Syria.
Changes on the ground in Syria have given credence to the possibility of cease-fire zones as outlined in the Russian proposal. On Tuesday, pro-government militias opened a new commercial corridor between government and rebel-held areas in the town of Khirbet Ghazaleh in southern Syria, imposing a tax of 20 percent. The tax essentially formalized smuggling routes that have profited militants on both sides and could presage the opening of routes in other areas.
Hisham Skeif, a former member of the opposition council in Aleppo and now a political spokesman for a rebel faction, was skeptical of the Russian proposal, saying it needed clarification on the precise boundaries of the cease-fire zones and the identities of the monitoring forces.
“It was thrown by the Russians as a step in the air,” he said. Russia and the government have typically described rebel fighters as jihadists as a justification to bomb them, he said, “so we are back to the same vortex.”