GENEVA - The United States and Russia on Saturday reached an agreement to eliminate Syria's chemical weapons, giving President Bashar al-Assad one week to reveal what kind of weapons his country has and where they are being kept.
The agreement also calls for what one U.S. official called an "ambitious" timeline for dealing with Syria's chemical weapons, setting a November deadline for eliminating that country's ability to manufacture and mix the weapons and calling for the destruction of all materials that could be used to make such weapons by the middle of next year.
Under the agreement, inspectors from the Organization for the Prohibition of Chemical Weapons, the international body that monitors compliance with chemical weapons bans, would have "immediate and unfettered access to inspect any and all sites in Syria." Their initial inspections are to be completed in November.
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."
President Barack Obama welcomed the U.S.-Russian agreement, calling it an "important, concrete step" toward the goal of destroying the weapons. He warned, however, that "if diplomacy fails, the United States remains prepared to act."
But the prospect of a U.S. military strike, which seemed just hours away only two weeks ago, now appears remote.
The agreement suggests that a military strike could be authorized "in the event of non-compliance," but that would come only after approval by the U.N. Security Council, where Russia holds a veto. In remarks before reporters here, Russian Foreign Minister Sergey Lavrov said such intervention was off the table for now.
Senior Obama administration officials said they expected a U.N. resolution in some form to pass within weeks of a Geneva agreement.
One possible course of action, they said, is the internationally verified transfer of Syria's chemical stockpiles to Russia, where they eventually would be destroyed.
There was no immediate reaction from Syria or Mr. Assad. In interviews on Russian television last week, Mr. Assad had said Syria would reveal the details of its chemical weapons stockpiles a month after its formal accession to the Convention on Chemical Weapons, the treaty that bans their existence.
But it seemed unlikely that he would openly resist a timeline agreed to publicly by Russia, the principal provider of his military's armaments.
Syrian opposition figures decried the agreement, saying it failed to hold Mr. Assad accountable for the deaths of hundreds of civilians in chemical weapons attacks. But defected Gen. Salim Idriss, who heads the U.S.-backed Syrian Military Council, said his group would "facilitate" the work of inspectors and would hold its fire when inspectors pass through government-held areas where rebels are fighting. He said under no circumstances, however, would the rebels observe a general cease-fire.
"In regions under our control, there are no chemical weapons," he said. "I don't know if this will just mean that inspectors will pass through the regions that are under rebel control. We are ready."
Unknown was what the response would be of other rebel groups, including the Nusra Front and the Islamic State of Iraq and the Levant, both of which are affiliated with al-Qaida and which comprise the Syrian rebels' most effective fighters.
Magnus Ranstorp, an expert on terrorism at the Swedish Defense College, said in an email that both groups bear no love for the United States, Russia or the United Nations and could target the chemical weapons control effort, especially rebels from Central Asia and the Caucasus who have fought bitter battles against Russian influence in Chechnya and elsewhere.
The agreement, spelled out in a four-page document that included two annexes, marked a remarkable development in the 30-month Syrian crisis that has been characterized not only by a brutal conflict that has killed more than 100,000 people from both sides and displaced 6.5 million from their homes, but also by a seemingly insurmountable impasse between the United States and Russia over how the conflict should be resolved.
Only two weeks ago, the United States appeared to be just hours away from sending cruise missiles into Syria in retaliation for an Aug. 21 alleged chemical weapons attack that killed hundreds of people, and Russia was staunchly defending its Syrian client, insisting that the rebels, not the government, were responsible.
At one point, Russian President Vladimir Putin called U.S. Secretary of State John Kerry a "liar" and last week he wrote a column that appeared in the New York Times accusing the United States of violating international law for its Syrian threats.world
New York Times and Washington Post contributed.