The agreement was reached after 10 months of failed attempts to halt the fighting and of suspended efforts for a political settlement in a conflict that began more than five years ago, has left nearly a half-million people dead and created the largest refugee crisis since World War II.
Secretary of State John Kerry and his Russian counterpart, Sergey V. Lavrov, announced the agreement in Geneva after weeks of negotiations that were marred, in President Obama’s words, by deep “mistrust” between the Russians and Americans.
It came at a time when relations between the United States and Russia, which have worsened throughout much of the Obama administration, have been especially jolted by accusations of Russian hacking and subterfuge in American politics. The tensions have been further exacerbated by the effusive praise for President Vladimir V. Putin of Russia by the Republican presidential nominee, Donald J. Trump.
The new arrangement on Syria is supposed to begin Monday, with a seven-day continuous “genuine reduction of violence,” in Mr. Kerry’s words, and broad, unrestricted humanitarian access to the ravaged northern city of Aleppo and other besieged areas.
If that works for the initial period, the United States and Russia are supposed to then establish a Joint Implementation Center, where they will share targeting data, and begin to bomb militants of the Nusra Front and the Islamic State.
The key element is that Russia is then supposed to restrain the forces of President Bashar al-Assad of Syria from conducting any air operations over Nusra and opposition areas, which the United States hopes will bring an end to the dropping of barrel bombs — including chlorine gas attacks — that have punctuated the brutality in the conflict.
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In return, the United States is supposed to persuade the opposition groups it has been supporting to separate themselves from Nusra forces. Mr. Assad has attacked many of them on the pretense of attacking Nusra Front fighters.
Mr. Kerry sounded extremely cautious about whether this new arrangement would work.
“We believe the plan, if implemented, if followed, has the ability to provide a turning point, a change,” he said. Sounding far more cautious here than he did in Munich in February when he announced an earlier “cessation of hostilities” that failed, he said: “No one is basing this on trust. We are basing it on oversight and compliance.”
The accord was reached after sharp divisions inside the Obama administration over the wisdom of sharing targeting information with Russia, and accusations that the Russians have used the negotiating period to help Mr. Assad regain control in Aleppo and strike at American-backed opposition groups.
Secretary of Defense Ashton B. Carter has been among the most vocal of the skeptics, saying this week in Britain that “Russia entered the Syrian tragedy saying it wanted to counter terrorism and end the civil war, which is the source of so much suffering, through a political transition.”
“What it has done is very different from what it said,” Mr. Carter said. “Unfortunately, so far, Russia with its support for the Assad regime has made the situation in Syria more dangerous, more prolonged, more violent.”
For Mr. Obama, who asked Mr. Kerry to keep working on the negotiation after he failed to reach an accord with Mr. Putin during the Group of 20 summit meeting in China last weekend, the new accord poses considerable risks.
For example, if the bombing of Nusra sites results in civilian casualties — which is almost inevitable given how militant extremist groups and civilians are closely located — there are bound to be accusations about who is responsible. Moreover, Pentagon officials are concerned that Russia will use the targeting data to learn more about how American forces identify and attack targets, at a moment when forces from the two countries are often in close proximity around Europe.
Mr. Lavrov took a few shots at the United States even while celebrating the arrangement, denouncing “arrogant sanctions” levied against Russia for its annexation of Crimea two years ago.
Earlier in the day, he suggested the United States could not decide on taking the deal, sending pizza and vodka to reporters to ease the wait, and coming by to joke about how long it took Mr. Obama and his team to make decisions.
But for Mr. Kerry, reaching this deal has become a personal mission, one that at times put him in conflict with the White House. He has pressed for a stronger military commitment in Syria and support for some opposition groups, along with a series of more aggressive covert actions, according to administration officials. Mr. Obama has been reluctant, as have others in the White House who fear that, even if they could engineer a transition in Syria, it could create a power vacuum that Iran, Russia and militant terrorist groups could exploit.
Among the Syrians, the plan was greeted with skepticism on all sides, particularly from armed opposition groups and their supporters, who, broadly speaking, have come to believe that the United States has lost interest in ousting Mr. Assad, and is willing to see them wiped out.
It is a measure of how little trust the Syrians have in the international community — especially after the short-lived cease-fire in February — that initial reactions were lukewarm, even though the deal holds out the possibility of at least a temporary calming of the violence.
Armed opposition groups read the deal as ordering them to remove better-armed Nusra fighters from their areas, something they lack the military power to do alone, or else face attack by the United States — a country that has provided some of the rebel groups with training and weapons for years.
While Mr. Kerry began his announcement by noting that Mr. Assad’s airstrikes were, as he put it, “the main driver of civilian casualties and migration flows,” the deal — as partly described — contains many loopholes that could allow them to continue.
And no measures were described that would hold any of the parties to account if they violated the terms of a deal that is being struck at a time when the United States has little leverage over Russia in Syria.
The deal allows Syrian government warplanes to continue to fly missions in some areas that are to be defined later.
And Russia and the United States will target areas where they both agree Nusra or Islamic State fighters are present. What that really means hinges on how Russia and the United States define legitimate opposition groups that cannot be targeted under the deal, and how they define areas where Nusra is present.
The deal also failed to mention anything about the presence of foreign Shiite militias — such as Hezbollah, which like Al Qaeda and Islamic State is considered a terrorist group by the United States — fighting on the Syrian government’s side.
It also said nothing about the tens of thousands of detainees in Syrian government prisons, whose release had long been touted as a possible measure under a deal. And while it spoke of allowing aid deliveries into besieged areas, it said nothing of lifting the sieges and restoring freedom of movement of goods and people.
Correction: September 9, 2016
An earlier version of this article misstated when the new arrangement between Russia and the United States is supposed to begin in Syria. The timing is Monday, not Sunday evening.