GENEVA -- The United States and Russia reached a sweeping agreement on Saturday that called for Syria's arsenal of chemical weapons to be removed or destroyed by the middle of 2014 and indefinitely stalled the prospect of American airstrikes.
However, the joint announcement, on the third day of intensive talks in Geneva, also set the stage for one of the most challenging undertakings in the history of arms control.
"This situation has no precedent," said Amy E. Smithson, an expert on chemical weapons at the James Martin Center for Nonproliferation Studies. "They are cramming what would probably be five or six years' worth of work into a period of several months, and they are undertaking this in an extremely difficult security environment due to the ongoing civil war."
Although the agreement explicitly includes the United Nations Security Council for the first time in determining possible international action in Syria, Russia has maintained its opposition to any military action.
But George Little, the Pentagon press secretary, emphasized that the possibility of unilateral American military force was still on the table. "We haven't made any changes to our force posture to this point," Mr. Little said. "The credible threat of military force has been key to driving diplomatic progress, and it's important that the Assad regime lives up to its obligations under the framework agreement."
In Syria, the state news agency, SANA, voiced cautious approval of the Russian and American deal, calling it "a starting point," though the government issued no immediate statement about its willingness to implement the agreement.
In any case, the deal represented at least a temporary reprieve for President Bashar al-Assad and his Syrian government, and it formally placed international decision-making about Syria into the purview of Russia, one of Mr. Assad's staunchest supporters and military suppliers.
That reality was bitterly seized on by the fractured Syrian rebel forces, most of which have pleaded for American airstrikes. Gen. Salim Idris, the head of the Western-backed rebels' nominal military command, the Supreme Military Council, denounced the initiative.
"All of this initiative does not interest us. Russia is a partner with the regime in killing the Syrian people," he told reporters in Istanbul. "A crime against humanity has been committed, and there is not any mention of accountability."
An immediate test of the viability of the accord will come within a week, when the Syrian government is to provide a "comprehensive listing" of its chemical arsenal. That list is to include the types and quantities of Syria's poison gas, the chemical munitions it possesses, and the location of its storage, production and research sites.
"The real final responsibility here is Syrian," a senior Obama administration official said of the deal.
Speaking at a joint news conference with his Russian counterpart, Secretary of State John Kerry said that "if fully implemented, this framework can provide greater protection and security to the world."
If Mr. Assad fails to comply with the agreement, the issue would be referred to the United Nations Security Council, where the violations would be taken up under Chapter VII of the United Nations Charter, which authorizes punitive action, Mr. Kerry said.
Foreign Minister Sergey V. Lavrov of Russia made clear that his country, which wields a veto in the Security Council, had not withdrawn its objections to the use of force.
If the Russians objected to punishing Syrian noncompliance with military action, however, the United States would still have the option of acting without the Security Council's approval.
The issue of removing Syria's chemical arms broke into the open on Monday when Mr. Kerry, at a news conference in London, posed the question of whether Mr. Assad could rapidly be disarmed, only to state that he did not see how it could be done.
Now, however, what once seemed impossible has become a plan -- one that will depend on Mr. Assad's cooperation and that will need to be put in place in the middle of a fierce conflict.
To hammer out the agreement, arms control officials on both sides worked into the night, a process that recalled treaty negotiations during the cold war.
Mr. Kerry and Mr. Lavrov held a marathon series of meetings on Friday, including a session that ended at midnight. On Saturday morning, the two sides reconvened with their arms control experts on the hotel pool deck as they pored over the text of the agreement.
Obama administration officials say that Russia's role is critical since it has been a major backer of the Assad government, and the American assumption is that much, if not all, of the accord has Mr. Assad's assent.
At the United Nations, Ban Ki-moon, the secretary general, pledged to support the agreement, and he announced that Syria had also formally acceded to the international Chemical Weapons Convention, effective Oct. 14.
Foreign Secretary William Hague of Britain issued a statement after a call with Mr. Kerry in which he welcomed the framework agreement on Syrian chemical weapons, describing it as a "a significant step forward."
It was a British Parliamentary vote against military action that put off momentum by the United States, France and Britain to conduct airstrikes in the wake of the August chemical strike within Syria.
"The priority must now be full and prompt implementation of the agreement, to ensure the transfer of Syria's chemical weapons to international control," Mr. Hague said. Under the agreement, titled "Framework For Elimination of Syrian Chemical Weapons," an inspection of the chemical weapons sites identified by the Syrian government must be completed by November. Equipment for producing chemical weapons and filling munitions with poison gas must be destroyed by November.
The document also says that there is to be "complete elimination of all chemical weapons material and equipment in the first half of 2014."
A priority under the agreement reached Saturday is to take steps to preclude or diminish the Assad government's ability to employ chemical weapons before they are destroyed.
An American official said that such steps could include burning the least volatile component of binary weapons, a type of chemical agent that becomes potent only when separate elements are mixed. Another way to disable at least part of Syria's stockpile, the official said, would be to destroy the equipment for mixing the binary component or destroying the munitions or bombs that would be filled with chemical agents.
An American official, who spoke on the condition of anonymity under State Department protocol, said that the United States and Russia had agreed that Syria has 1,000 tons of chemical weapons, including sarin and mustard gas.
The United States believes there are at least 45 sites in Syria associated with its chemical weapons program. Nearly half of these have "exploitable quantities" of chemical weapons, though the American official said that some of the agents may have been moved by the Assad government.
The American official said there was no indication that any of Syria's chemical stocks had been moved to Iraq or Lebanon, as the Syrian opposition had charged. "We believe they are under regime control," the official said.
Russia has not accepted the American data on the number of chemical weapons sites. The difference may reflect the larger disagreement as to who was responsible for an Aug. 21 attack that the United States says killed at least 1,400 civilians, many of them women and children.
If the Russians were to agree both on the number of chemical weapons sites and that the sites are all in government-controlled areas, that would suggest that the Assad government was culpable for the attack, and not the rebel forces as the Russians have asserted.
The four-page framework agreement, including its technical annexes, are to be incorporated in a Security Council resolution that is to be adopted in New York.
One concern in carrying out the deal, however, involves how to protect international inspectors who go to Syria. There will be no cease-fire so the inspectors can carry out their work.
Asked whether rebels would aid the inspectors, General Idris, the Western-backed rebel military commander, called the issue "complicated," saying, "If investigators come, we will facilitate the mission."
He said there were no chemical weapons in rebel-controlled areas, adding: "I don't know if this will just mean that investigators will pass through the regions that are under rebel control. We are ready."
The sense of betrayal among nominally pro-Western factions in the opposition has grown intensely in recent days.
In the northern Syrian province of Idlib, a rebel stronghold, one commander said that the agreement on Saturday proved that the United States no longer cared about helping Syrians and was leaving them at the mercy of a government backed by powerful allies in Russia and Iran.
Maysara, a commander of a battalion in Saraqeb, said in an interview that he had paid little attention to the diplomacy on Saturday.
"I don't care about deals anymore," he said in an interview. "The Americans found a way out of the strike."
He added: "The Russians did what they want. The Americans lied, and the lie was believed -- the U.S. doesn't want democracy in Syria. Now I have doubts about the U.S. capacities, their military and intelligence capacities. The Iranian capacity is much stronger, I guess."
Peter Baker contributed reporting from Washington, and Anne Barnard from Beirut, Lebanon.
This article originally appeared in The New York Times.