"President Obama has made clear that should diplomacy fail force might be necessary to deter and degrade Assad's capacity to deliver these weapons," Kerry said in Geneva at the start of talks with Russian Foreign Minister Sergei Lavrov.
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Kerry is accompanied by a large retinue of experts in anticipation of detailed talks on how to turn the Russian offer into a concrete plan along the lines of disarmament accords between Washington and Moscow since the days of the Cold War.
The US delegation will present the Russians with US spy services' assessment of Syria's chemical arms infrastructure, said the US official travelling with Kerry.
Destroying chemical weapons in a war zone will be hard, the official added: "It is doable, but difficult and complicated."
"Expectations are high. They are high for the United States perhaps even more sides for Russia to deliver on the promise of this moment. This is not a game and I said that to my friend Sergei when we talked about it initially," Kerry said.
"It has to be real. It has to be comprehensive. It has to be verifiable. It has to be credible. It has to be timely and implemented in a timely fashion, and finally there ought to be consequences if it doesn't take place."
Obama, whose attention has been consumed by Syria since he threatened military strikes to punish Assad's government for a poison gas attack that killed hundreds of people in Damascus suburbs two weeks ago, said he was now turning to domestic priorities while backing Kerry's efforts.
This week's eleventh-hour Russian initiative interrupted a Western march to war, persuading Obama to put strikes on hold.
Syria, which denies it was behind that attack, has agreed to Moscow's proposal that it give up its chemical weapons stocks, averting what would have been the first direct Western intervention in a war that has killed more than 100,000 people.
"In the next couple of days, Syria will send a petition to the United Nations and Organization for the Prohibition of Chemical Weapons," Assad said in an interview on Russian TV.
"The petition will contain technical documents required to sign the agreement. After that, work will start that will lead to the signing of the convention prohibiting chemical weapons."
US officials said they hoped Kerry and Lavrov could agree on a blueprint with the main points to be adopted in a UN Security Council resolution. An initial French draft calls for an ultimatum to Assad's government to give up its chemical arsenal or face punitive measures.
The Russian initiative offers Obama a way out of a threat to use force, which is deeply unpopular among Americans after 12 years of war in Iraq and Afghanistan. Obama had asked Congress for authorization for strikes and faced a tough fight persuading skeptical lawmakers of the case. That vote is now on hold.
He told a cabinet meeting on Thursday he was now focusing on domestic priorities, while he was hopeful that Kerry's talks with Lavrov would "yield a concrete result".
The sudden pull-back from the brink is a blow for rebels who have listened to Obama and other Western leaders declare in strong terms for two years that Assad must be removed from power while wavering over whether to use force to push him out.
Rebels have long pleaded with the West for advanced weapons to counter Assad's firepower. Obama promised unspecified military aid in June; since then, Washington has trained rebel units but has not delivered arms.
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