President Barack Obama says an agreement between the United States and Russia offers a chance to destroy Syria's huge stockpile of chemical weapons and promises to end the threat the weapons pose to the region and the world as well as the Syrian people.
Obama says the international community expects Syria to live up to its public commitments to hand over its chemical weapons stockpile.
Warning that the US remains prepared to act if Syria falls short, he also cautions that more work remains even after the progress the deal represents.
''The use of chemical weapons anywhere in the world is an affront to human dignity and a threat to the security of people everywhere,'' Obama said in a statement Saturday.
''We have a duty to preserve a world free from the fear of chemical weapons for our children. Today marks an important step towards achieving this goal.''
In setting out one of the most ambitious arms-control efforts in history, US and Russian officials reached an agreement calling for an inventory of Syria's chemical weapons program and seizing all its components.
The agreement includes imposing penalties if Syrian President Bashar Assad's government fails to comply.
After three days of talks in Geneva, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov demanded Assad account for his secret stockpile within a week and let international inspectors eliminate all the weapons by the middle of next year - an "ambitious" target, Kerry said.
The accord leaves major questions unanswered, including how feasible such a major disarmament can be in the midst of civil war and at what point Washington might yet make good on a continued threat to attack if it thinks Assad is reneging.
Under the Geneva pact, the United States and Russia will back a United Nations enforcement mechanism. But its terms are not yet set. Russia is unlikely to support the military option that President Barack Obama said he was still ready to use.
"If diplomacy fails, the United States remains prepared to act," Obama said. "The international community expects the Assad regime to live up to its public commitments."
But for Assad's opponents, who two weeks ago thought US air strikes were imminent in response to a gas attack on rebel territory, the deal was a blow to hopes of swinging the war their way. Kerry and Lavrov said it could herald broader peace talks, as warplanes hit rebel positions again near Damascus.
The accord, however, was as much about US-Russian ties as it was about Syria. The conflict has chilled relations to levels recalling the Cold War.
In reaching a bilateral deal Saturday (local time) after what one US official described as three days of "hard fought" debate, Moscow and Washington can each count benefits.
For Russian President Vladimir Putin, it brings management of the Syrian crisis back to the United Nations. For Obama, it solves the dilemma created by Congress's reluctance to back military strikes that he was preparing without a UN mandate.
Lavrov told a joint news conference in Geneva: "It shows that when there is a will ... Russia and the United States can get results on the most important problems."
"The successful realisation of this agreement will have meaning not only from the point of view of the common goal of eliminating all arsenals of chemical weapons, but also to avoid the military scenario that would be catastrophic for this region and international relations on the whole."
Kerry acknowledged, however, that further success was far from guaranteed: "The implementation of this framework, which will require the vigilance and the investment of the international community, and full accountability of the Assad regime, presents a hard road ahead," he said.
Despite a measure of camaraderie on display in banter between the two men during the presentation of the two-page framework agreement, they remained openly at odds over the US willingness to use force in Syria without UN backing.
In Istanbul, the head of the Syrian rebel Supreme Military Council, General Selim Idris, called it a blow to opposition hopes of overthrowing Assad and accused the Syrian president of circumventing any disarmament by already sending chemical weapons to allies in Lebanon and Iraq in recent days.
Qassim Saadeddine, a rebel commander in northern Syria, told Reuters: "Let the Kerry-Lavrov plan go to hell. We reject it and we will not protect the inspectors."
Idris, however, said his forces would help the inspectors. A US official said Washington believed all Syria's chemical weapons remained in areas under government control.
Assad, backed by Iran, crushed demonstrations demanding democracy in 2011 and, while losing territory, has withstood a full-blown uprising supported by Arab states and the West.
Prodded by Russia, he has now agreed to abide by a global chemical weapons ban and submit to controls by the UN-backed Organisation for the Prohibition of Chemical Weapons (OPCW).
That will deprive him of arms which he denies having used and spares him, at least for now, what were likely to be heavy US and French missile strikes and bombing raids.
Assad's opponents criticise the United States and its allies for the focus on chemical weapons, which account for a very small number of the more than 100,000 dead in the war, and of abandoning them to face a deadly, Russian-armed enemy.
On Saturday, as government jets struck rebel-held suburbs of the capital and troops fought rebels on the ground, one opposition activist in Damascus voiced anger and resignation:
"The most important point is the act of killing, no matter what is the weapon," said Tariq al-Dimashqi. "The killing will continue. No change will happen. That is it."
Syrian state media broadcast the Geneva news conference live, indicating that Damascus is satisfied with the deal.
DEAL OFFERS WAY OUT
Having taken the surprise decision two weeks ago to seek congressional approval for strikes to punish Assad for using poison gas, Obama faced a dilemma when lawmakers appeared likely to deny him.
They cited unease about helping Islamist militants among the rebels and a wariness of new entanglements in the Middle East after wars in Iraq and Afghanistan.
The weapons deal proposed by Putin, a former KGB agent intent on restoring some of the influence Moscow lost with the Soviet collapse two decades ago, offered Obama a way out.
Russia has protected and armed Assad and has been alarmed at what it sees as Western willingness to bypass the United Nations to impose "regime change" in other states.
Under the terms of the US-Russian agreement, the UN Security Council - on which Russia has a veto - will oversee the process. Syria must let the OPCW complete an initial inspection of its chemical weapons sites by November.
Kerry said Assad must produce a "comprehensive listing" of its chemical weapons within a week. The goal is to complete destruction of Syria's arsenal in the first half of 2014.
The agreement states that a Security Council resolution should allow for regular assessments of Syria's behaviour and "in the event of non-compliance ... the UN Security Council should impose measures under Chapter VII of the UN Charter".
Chapter VII can include force but can be limited to other kinds of sanction. When Kerry said the Council "must" impose measures under Chapter VII, Lavrov interrupted to point out that the agreed text says only it "should" impose penalties.
"There's no diminution of options," Kerry said, noting Obama's right under US law to order military action, with or without support from Congress or any international body.
The Pentagon said it had made no change to the forces it had lined up. A spokesman said: "The credible threat of military force has been key to driving diplomatic progress."
Lavrov said of the agreement: "There is nothing said about the use of force and not about any automatic sanctions."
Putin has supported Assad's contention that the sarin gas attack on August 21, which Washington says killed over 1,400 civilians, was the work of rebels trying to provoke Western intervention.
Lavrov said, however, that Russia would support UN punishment for anyone whose guilt was clearly proven.
A new round of argument about responsibility for the August 21 deaths is likely in the coming days, once UN inspectors deliver their report on the incident to the world body.
In two weeks, when the UN General Assembly convenes in New York, Lavrov and Kerry have said they will meet the UN. Syria envoy to see if they can push forward a plan for an international peace conference to negotiate an end to the war.
An effort last year for a political solution, dubbed the "Geneva Plan" and calling for a transitional government, went nowhere as Assad refused to cede power and the opposition insisted he could not be a part of any new political order.
Kerry said Saturday's chemical weapons deal could be "the first concrete step" toward a final settlement. Lavrov said he hoped all parties to the conflict could attend a conference in October, without setting pre-conditions for their attendance.
There is little sign of softening among Syrians, however, or among rival backers in the Middle East, where the conflict is part of a broader regional confrontation with sectarian elements between Shi'ite Iran and Sunni Muslim Arab leaders.
The opposition Syrian National Coalition in Istanbul elected a moderate Islamist, former political prisoner Ahmad Tumeh, as its provisional prime minister on Saturday. Members said they hoped Tumeh could help make the often fractious opposition's case in negotiations to achieve their demands.
Senior Kerry aides involved in the talks said that the United States and Russia agreed that Syria has 1,000 tonnes of chemical agents, including nerve gas sarin and mustard gas - one of the world's largest stockpiles of such material.
But the officials said there was no agreement on how many sites must be inspected. Washington thinks it is at least 45.
One US official called the task "daunting to say the least". Another noted there were "targets ... not a deadline".
The weapons are likely to be removed through a combination of destroying them in Syria and shipping some for destruction elsewhere, US officials said. Russia is one possible site for destruction, but no final decisions have been made.
The Hague-based OPCW has never moved weapons across borders before, due to the risk, and never work in a war zone.
Burning vast quantities of toxic chemicals alone is time consuming and risky. So too is separating them from ammunition in pre-loaded weaponry. Chemicals have to be injected in small quantities into a furnace and burned, while filtering fumes.