WASHINGTON -- 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 White House declined to comment on the assessments, kept under wraps amid intense U.N. negotiations on a plan for dismantling Syria's chemical stockpile. The five permanent U.N. Security Council members agreed Thursday on a resolution that requires Syria to surrender its chemical weapons.
Senior U.S., Russian, British and French diplomats confirmed the accord, which also includes China. Britain's U.N. Ambassador Mark Lyall Grant intended to introduce the text to the Security Council's 10 non-permanent members Thursday night.
A vote on the resolution still depends on how the full council responds to the draft, and on how soon an international group that oversees the global treaty on chemical weapons can adopt a plan for securing and destroying Syria's stockpile. Diplomats said the earliest the Security Council could vote would be late today.
On Twitter, Mr. Lyall Grant said the five veto-wielding members, known as the P-5, had agreed on a "binding and enforceable draft .... resolution." A senior U.S. State Department official said the Russians agreed to support "a strong binding and enforceable resolution."
But the draft, seen by The Associated Press, makes clear that there is no trigger for any enforcement measures if Syria fails to comply with the resolution's provisions or the dismantling of its chemical weapons stockpile. Instead, it states that in the event of non-compliance, or any use of chemical arms, the Security Council will "impose measures under Chapter 7 of the United Nations Charter," which will require a second resolution.
Chapter 7 allows for military and nonmilitary actions to promote peace and security. Russia, Syria's most powerful ally, had opposed any reference to it. U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov met in hastily scheduled, closed-door talks Thursday afternoon to resolve several last-minute disputes on the text, and the agreement was announced soon after.
The insights into Syria's arsenal have been bolstered by the Damascus government's own accounting, which lists the types of chemical agents and delivery systems it possesses. That 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 said.
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.
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 Russian and U.S. intelligence agencies had independently reached similar conclusions about the size of Syria's chemical arms program, seen 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 rest consists of chemical precursors of the nerve agent sarin, said to be "unweaponized" and in "liquid bulk" form, according to two people who attended White House briefings.
Weapons experts not privy to the briefings described those 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 Cold War-era U.S. 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 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," Mr. 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 an Aug. 21 sarin attack that killed more than 1,000 civilians in two Damascus suburbs. U.S. surveillance systems observed Syrian troops mixing chemical precursors three days before sarin-filled rockets exploded in one suburb.
White House officials say U.S. and Russian officials were mostly in accord on the nature of Syria's chemical arsenal and how to dismantle it, said the two experts who attended the briefings.
The two nations did not agree, however, on the number of Syrian chemical munitions storage sites, and differed, too, over where the physical destruction of sarin and other toxins should take place.
The Obama administration prefers to remove all chemical weapons from Syria as soon 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 briefingss. He spoke on condition of anonymity to discuss that.
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 nations expressed optimism that Syria will comply with U.N. demands to surrender its chemical arms. 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.
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.
Associated Press contributed.