Significantly, the agreement to establish the de-escalation zones has not been accepted by all opposition groups, and it left loopholes for Syria to continue undiminished attacks on the rebels, factors that combined to hinder the last attempt by Iran, Russia and Turkey to forge a cease-fire.
Nevertheless, one of the representatives of the Syrian opposition groups at talks in Astana, Kazakhstan, Col. Ahmad Berri, sounded a strikingly optimistic note, saying he expected to see a full cease-fire in the designated zones once the plan takes effect on Saturday.
“The Russians this time are more serious, we sensed it, more than last time,” he said in a telephone interview. “The regime will be committed to the deal because the Russians are the guarantor, so if the Russians said no bombing, the regime will stop.”
No-fly zones have been a contentious issue in the Syrian conflict, now in its seventh year; they have long been requested by rebel groups and rejected by the government. Disputes about who can fly planes and when — “subtle professional issues,” the Russian president, Vladimir V. Putin, called them recently — are likely to continue under the new deal.
Confusion about the details of agreements in the Syrian civil war is nothing new, as disputes about the most basic facts and definitions have always made negotiations extraordinarily thorny.
The government of President Bashar al-Assad of Syria said in a statement late Wednesday night that it “supports” the initiative on de-escalation zones, “including not shelling those areas.”
But the statement also said the Syrian military would continue to fight banned terrorist groups like the Islamic State, Qaeda-linked militants “and other affiliated terrorist organizations wherever they were all over the Syrian territories.” That language was interpreted by many government opponents as a signal that the Syrian military intended to keep bombing wherever it chose on the pretext of fighting terrorism.
“Aviation over these territories ceases,” Mr. Lavrentiev told reporters in Astana on Friday, where a memorandum laying out the plan for de-escalation zones was signed the day before between Iran and Russia, which back the Syrian government, and Turkey, which backs some rebel groups.
But in answer to a question about the United States-led coalition formed in 2014 to fight the Islamic State, Mr. Lavrentiev did not mince words: “The work of aviation, especially the forces of the international coalition, is absolutely not envisaged. With notification or without notification, this issue is now closed.”
He added: “The only place where the international coalition’s aviation can work is on the objects of the Islamic State that are located in the Raqqa area, some populated areas in the area of the Euphrates, Deir al-Zour and further on the Iraqi territory.”
That suggests that the American military would no longer be allowed to fly over a number of critical areas where it already conducts operations and that it would be barred from all of the most important areas contested by the government and rebels not affiliated with the Islamic State.
The excluded area encompasses Idlib Province, where American warplanes have been carrying out an intensifying series of airstrikes against what officials say are Qaeda operatives. It also includes some of the areas where Turkey, a NATO ally, has skirmished with Kurdish militias also backed, sometimes with airstrikes, by the United States.
And it includes most of the Syrian government’s military installations, such as the Shayrat air base, which the United States struck with missiles in retaliation for chemical attacks that killed scores of people in the rebel-held town of Khan Sheikhoun in Idlib last month.
The de-escalation zones are along Syria’s southern border with Jordan, in the eastern Damascus suburbs, in Idlib Province and in a pocket of the central province of Homs.
In some ways, such discussions are academic. The United States has never had Syrian government permission for its airstrikes on the Islamic State and on Qaeda targets on Syrian territory. The Syrian government calls the American strikes violations of its sovereignty.
And Russia and the Syrian government have liberally interpreted exceptions to previous cease-fire deals, continuing to carry out strikes, including some that hit rescue workers and hospitals and that were followed by declarations from Moscow and Damascus that terrorist groups had been present in the areas targeted.
Rebel groups, too, have argued about the meaning of provisions requiring them to separate from banned terrorist groups, asserting that they lack the ability to push out well-funded and well-armed extremists. But, at the same time, some rebel groups have entered into tactical alliances with the extremists.
Still, United Nations officials have held out hope that the new deal will fare better than others that have evaporated under the weight of those contradictions. American officials said they shared the stated goals of the plan but expressed skepticism that Russia could restrain the Syrian government and concern about the role of Iran.
What makes this agreement different is that some of the countries backing different sides in Syria have agreed at least on the possibility of bringing in outside military forces to monitor a cease-fire. Those could be United Nations peacekeepers, troops from Arab countries, or forces from countries friendly to both Russia and Turkey, like the former Soviet republics.
Some analysts, on both the government and on the opposition sides, have complained about precisely those attributes of the agreement: that it could be a first step to the de facto division of the country along existing conflict lines and that it is being imposed by outside powers.
Mr. Putin said on Wednesday that aircraft would not operate over the designated zones, “provided that these zones show no sign of military activity.”
“These are all subtle professional issues,” he said.
Government opponents said that a real end to bombings across the country was the top demand of their supporters.
But they saw the deal as something else: a pretext to make sure that there would be no repeat of the strikes ordered by President Trump and to shore up Mr. Assad politically. They said the latest deal recalled the agreement to remove Syria’s chemical weapons that Russia and the United States struck as an alternative to punitive strikes for chemical attacks in 2013.
“This deal is like when the Russians rescued Bashar in 2013, and now they’re trying to rescue him again with plastic surgery agreements,” said Hisham Marwa, a member of an opposition coalition.