The countdown has begun on a dangerous, tough mission to eliminate Syria's chemical weapons, with the regime given just seven days to lift the veil on its secret stockpile and allow inspectors into the war-torn country.
In whirlwind, round-the-clock diplomacy in Geneva, the United States and Russia hammered out a landmark framework for dismantling and destroying one of the world's largest stockpiles of chemical arms.
By next Friday President Bashar Al-Assad faces his first litmus test of the seriousness of his regime's stated commitment to bring its deadly chemical arms under international control.
The deal hammered out in Geneva has to go first to the Organisation for the Prohibition of Chemical Weapons to be agreed, but within seven days Assad must put on the table a full declaration of the extent of his long secret programme.
The United States believes Syria is dotted with some 45 sites connected to the chemical weapons programme – of which nearly half contain "exploitable chemical agents" for mixing toxic gasses.
Questions remain over whether the US and Russia can meet their goal of removing and destroying some 1,000 metric tonnes by mid-2014, but US experts emerging from three days of non-stop talks with Russian counterparts insist it can be done.
"It's very, very difficult, but it's doable," said one US administration official, with another calling it an "ambitious" timetable.
Under the plan, arms inspectors and experts must be given immediate access to the sites in question to begin neutralising them and locking them down. And all initial inspections of sites are due to be completed by November.
One of the most difficult issues though, is that Syria is in the middle of a brutal civil war, which has cost some 110,000 lives since March 2011.
US officials believe that despite the chaotic battle on the ground, the Assad regime has kept tight control of the chemical weapons stocks and they should have relatively easy access to regime-controlled areas.
"The security is still a daunting challenge," said the administration official, adding Pentagon experts have already drawn up options for how weapons experts could protect themselves and the arms sites.
Harnessing enough manpower could also prove difficult, with the United States and Russia committing to try to drum up help – and funding – from allies.
"Personnel should be dispatched as soon as possible," said the official, adding they may draw on the expertise of the five permanent members of the UN Security Council.
Many details still need to be worked out, including whether the stocks would be destroyed on site or moved to third country for destruction.
"There are clear advantages and disadvantages to removing some of the agents from Syria. It is potentially faster," said another US official.
"It also has certain risks and costs any time you move weapons or chemicals of this sort," he said, adding it would be premature to identify any of the countries which could be used.
The United States and Russia are the leading experts in the field, having moved to destroy all their chemical weapons under a 1997 deal, and have even developed a mobile destruction unit which can be placed in a country without the proper facilities.
Some chemical weapons are destroyed through a process known as hydrolysis, in which agents, like detergents, are used to neutralize blistering chemicals such as mustard gas and sulphur.
Nerve gases such as sarin are often better destroyed through incineration.
"Production equipment of course is separate from the actual chemicals and there you need a big sledgehammer and some other equipment," the second US official said.
But the exhausted negotiators were exhilarated that in less than three days, they had managed to pull off a deal, which they say gives them a real chance of eliminating Syria's chemical weapons.
The speed at which it all came together was also noteworthy.
Earlier this week, the Russian ambassador to Washington came in "and brought us some ideas, and we had some ideas, but no-one had a full blown plan. No-one," the first US official said.
The Geneva "meeting was put together logistically in 24 hours, so people had to create all of this here," she added.
Apart from their weapons experts and intelligence and security staff, both sides also brought their lawyers.
"At the end of the day the things that most mattered I believed that we got," she said, asked if the US had had to compromise too much.
"Did they agree to every single word we wanted? No. Did we have hard-fought negotiations, yes, or we could have done this in an hour. But we feel we were able to do this, to move forward and do what we set out to do."