US and Russian officials believe that the vast majority of Syria's nerve agent stockpile consists of "unweaponised" liquid precursors that could be neutralised relatively quickly.
That will lower the risk the toxins could be hidden away by the regime or stolen by terrorists.
A confidential assessment by the two governments also concludes that Syria's entire arsenal could be destroyed in about nine months, according to two people briefed on the analysis.
The assessment came as the five permanent members of the UN Security Council reportedly agreed on Thursday on the resolution requiring Syria to give up its chemical weapons to international inspectors, but with no automatic penalties if they fail to comply. The deal has yet to be approved by the full 15-member Security Council.
Experts from the world's chemical weapons watchdog will begin inspecting Syria's stockpile of toxic munitions by Tuesday, according to a draft agreement.
The draft, which was due to be voted on last night, calls on members of the Organisation for the Prohibition of Chemical Weapons (OPCW) to make cash donations to fund Syria's fast-tracked destruction operation.
UN experts in Syria are also investigating seven alleged chemical weapons attacks and expect to wind up their work on the ground on Monday, the United Nations said yesterday.
In a statement issued from the Syrian capital, where the team of experts arrived earlier this week, the UN said the experts hoped to have a comprehensive report ready "by late October".
Even as diplomacy gathered pace, fresh violence erupted in Syria, with a car bomb outside a mosque killing at least 30 people in Rankus, a Sunni town north of Damascus that backs the Sunni--dominated opposition to the government, the Syrian Observatory for Human Rights said.
The insights into the nature of Syria's chemical arsenal, thought to be the most authoritative to date, reflect the consensus view of Russian and US analysts who compared their governments' intelligence on Syria during meetings in Geneva this month. The Obama administration has since briefed independent experts on the key findings.
The Damascus government's own accounting of the arsenal, presented on Saturday to the Organisation for the Prohibition of Chemical Weapons in The Hague, generally corresponds to the US and Russian assessment.
US officials have reviewed the Syrian inventory, which has not been publicly released, and "found it quite good", a senior US State Department official said.
The deal struck on Thursday after days of negotiations is a compromise among the US, its allies and Russia about how to enforce the resolution.
When approved by the full Security Council, it would amount to the most significant international diplomatic initiative of the Syrian civil war.
Western diplomats said the resolution would be legally binding and would stipulate that if Syria failed to abide by the terms, the Security Council would take measures under Chapter VII of the UN Charter, the strongest form of a council resolution.
Such measures could include economic sanctions or even military action. But before any action could be taken, the issue would have to go back for further deliberations by the Security Council.
"This resolution makes clear there will be consequences for non-compliance," Samantha Power, the US ambassador to the United Nations, said.
In private briefings to weapons experts, White House officials said analysts had concluded that Syria had more than 1,000 tonnes of chemical weapons, of which about 300 tonnes were sulphur mustard, the blister agent used in the first world war.
Nearly all of the remainder consists of chemical precursors of the nerve agent sarin, described as being "unweaponised" and in "liquid bulk" form, according to people who attended White House briefings.
Experts not privy to the briefings described the findings as encouraging. Several noted that it is far easier to destroy precursor chemicals than battlefield-ready liquid sarin or warheads already loaded with the toxin.
"If the vast majority of it consists of precursors in bulk form, that is very good news," said Michael Kuhlman, chief scientist in the national security division at Battelle, a company that has destroyed much of the US cold war chemical stockpile.
Additional reporting by The New York Times, Bloomberg, Reuters