GENEVA — The United States and Russia reached an agreement Saturday that called for Syria’s arsenal of chemical weapons to be removed or destroyed by the middle of 2014 and that indefinitely stalled the prospect of U.S. airstrikes.

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 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 U.N. Security Council for the first time in determining possible international action in Syria, Russia has maintained its opposition to any military action.

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George Little, the Pentagon press secretary, emphasized that the possibility of unilateral U.S. military force was still on the table. “We haven’t made any changes to our force posture to this point,” Little said.

In Syria, the state news agency, SANA, voiced cautious approval of the Russian and U.S. deal, calling it “a starting point,” though the government issued no immediate statement about its willingness to implement the agreement.

The deal represented at least a temporary reprieve for President Bashar Assad and his Syrian government, and it formally placed international decision-making about Syria into the purview of Russia, one of 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 U.S. airstrikes. Gen. Salim Idris, 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 said in Istanbul. “A crime against humanity has been committed, and there is not any mention of accountability.”

The agreement, if successfully implemented, marks a modest victory for the Obama administration in its mostly arms-length engagement with Syria’s 2½-year civil war.

It was President Obama’s threat of military strikes after the U.S. said Syria’s government used chemical weapons in an Aug. 21 attack in the suburbs of Damascus that began the process that culminated in Saturday’s agreement. More than 100,000 people have died in the Syrian uprising since Obama in 2011 called for Assad to step down.

Obama said Saturday that the deal “represents an important, concrete step toward the goal of moving Syria’s chemical weapons under international control so that they may ultimately be destroyed.”

An immediate test of the accord will come this week, when the Syrian government is to provide a “comprehensive listing” of its chemical arsenal.

Speaking at a 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 Assad fails to comply, the issue would be referred to the U.N. Security Council, where the violations would be taken up under Chapter 7 of the U.N. Charter, which authorizes punitive action, Kerry said.

Foreign Minister Sergey Lavrov of Russia made clear that Russia, 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. “If diplomacy fails, the United States remains prepared to act,” Obama said.

Lavrov emphasized that the documents released Saturday, outlining the transfer of Syria’s chemical-weapons arsenal and its destruction, constitute only an “agreed proposal” that does not have the force of law.

Kerry and Lavrov held a marathon series of meetings that began Thursday. On Saturday morning, the two sides reconvened with their arms-controls experts on the hotel pool deck as they pored over the text of the agreement.

Obama administration officials say Russia’s role was critical because it has been a major backer of the Assad government, and the U.S. assumption is that much if not all of the accord has Assad’s assent.

At the United Nations, Secretary-General Ban Ki-moon pledged to support the agreement, and he said Syria had also formally acceded to the international Chemical Weapons Convention, effective Oct. 14.

Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina, who support greater U.S. assistance for Syria’s rebels, said the agreement will embolden enemies such as Iran.

“What concerns us most is that our friends and enemies will take the same lessons from this agreement: They see it as an act of provocative weakness on America’s part,” they said in a joint statement. “We cannot imagine a worse signal to send to Iran as it continues its push for a nuclear weapon.”

Under the agreement, an inspection of the chemical-weapons sites that the Syrian government declares must be completed by November. Equipment for producing chemical weapons and filling munitions with poison gas must be destroyed by November.

A U.S. official, who spoke on condition of anonymity, said 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. Russia has not accepted the U.S. data on the number of chemical-weapons sites.

Material from The Associated Press and The Washington Post is included in this report.