U.S. and Russian officials now believe that the vast majority of Syria's nerve agent stockpile consists of "unweaponized" liquid precursors that could be neutralized relatively quickly, lowering the risk that 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, assuming that Syrian officials honor promises to surrender control of its chemical assets to international inspectors, according to two people briefed on the analysis.
The assessment, thought to be the most authoritative to date, reflects the consensus view of Russian and U.S. 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 insights into Syria's arsenal have been bolstered further by the Damascus government's own accounting, which lists the types of chemical agents and delivery systems it possesses and was presented Saturday to the Organization for the Prohibition of Chemical Weapons in The Hague. U.S. officials have reviewed the Syrian inventory, which has not been publicly released, and "found it quite good," a senior State Department official told reporters.
In military weapons programs, two chemical precursors for sarin are blended using special equipment as the toxins are loaded into rockets, bombs or artillery shells.
The White House declined to comment on the assessments, which have been kept under wraps amid intense negotiations at the United Nations on a plan for dismantling Syria's chemical stockpile. The United Nations' five big powers reached agreement Thursday on a legally binding U.N. Security Council resolution that would require Syria to dismantle its once-secret chemical weapons program or face the threat of unspecified measures, according to senior U.S. and Russian officials.
Russia has long been a close ally and arms supplier to Syria and maintains strong ties to its military and intelligence services. Obama administration officials have said that Russian and U.S. intelligence agencies had independently reached similar conclusions about the size of Syria's chemical weapons program, regarded as one of the world's largest.
In private briefings to weapons experts, White House officials said analysts had concluded that Syria possesses more than 1,000 metric tons of chemical weapons, of which about 300 metric tons are sulfur mustard, the blister agent used in World War I. Nearly all of the remainder consists of chemical precursors of the nerve agent sarin, described as being "unweaponized" and in "liquid bulk" form, according to two people who attended White House briefings.
Weapons 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 supervised the destruction of much of the United States' Cold War-era chemical stockpile. "Now you're dealing with tanks of chemicals that are corrosive and dangerous, but not nerve agents. And the destruction processes for those chemicals are well in hand."
If U.N. inspection teams can remove even one of the sarin precursors - or the equipment used for measuring and filling - they can all but eliminate Syria's ability to launch a chemical attack even before the stockpile is completely destroyed, said Daryl Kimball, director of the Washington-based Arms Control Association.
"The mixing equipment itself is essential to using chemical agents," Kimball said. "If you prioritize the destruction of the equipment, you can largely deny Syria the ability to use these weapons again on Syrian soil."
Washington and Moscow have sparred repeatedly over Western allegations that Syria was behind a sarin attack that killed more than 1,000 civilians in two Damascus suburbs on Aug. 21.
U.S. surveillance systems observed Syrian troops mixing chemical precursors three days before sarin-filled rockets exploded in a Damascus suburb.
U.S. and Russian officials were mostly in agreement on the nature of Syria's arsenal and how to dismantle it, according to the two experts who attended the briefings.
The two countries did not agree on the number of storage sites for chemical munitions in Syria, and they differed on where the physical destruction of sarin and other toxins should take place. The Obama administration prefers to remove all chemical weapons from Syria as quickly as possible, before President Bashar Assad changes his mind, while Russia wants the weapons destroyed on Syrian soil, said a weapons expert who attended one of the briefings. He spoke on the condition of anonymity to discuss the briefing.
A senior Russian official said Thursday in Moscow that Russia was prepared to provide troops to guard the chemicals as they are being destroyed.
Both countries expressed optimism that Syria will comply with U.N. demands to surrender its chemical weapons. Syria's arsenal was initially developed as a deterrent to a future Israeli attack, but Assad may now view the weapons as a liability after the international outcry over the Aug. 21 attack, White House officials said at the briefings.
The apparent change of heart also could reflect discord within the Syrian government over the use of sarin, which some officials suspect may have been ordered by a senior regime official without Assad's authorization, the officials said.
In any event, they said, Assad is obliged to honor his promises or risk angering the Russian government and embarrassing President Vladimir Putin, who has given personal assurances that the weapons will be eliminated.
The White House briefers described the Russians as "serious and sincere," and even more prepared that the U.S. team in addressing the legal and technical hurdles involved in bringing Assad's arsenal under international control, one of the attendees said.
Washington Post staff writer Anne Gearan in New York contributed to this report.