U.S. intelligence believes Assad has about 45 sites associated with chemicals weapons, half of which have "exploitable quantities" of material that could be used in munitions. The Russian estimate is considerably lower; the officials would not say by how much.
U.S. intelligence agencies believe all the stocks remain in government control.
Noncompliance by the Assad regime or any other party would be referred to the 15-nation U.N. Security Council by the Organization for the Prohibition of Chemical Weapons. That group oversees the Chemical Weapons Convention, which Syria this past week agreed to join. The United Nations received Syria's formal notification Saturday. It would be effect Oct. 14.
The weapons group's director-general, Ahmet Uzumcu, spoke of adopting "necessary measures" to put in place "an accelerated program to verify the complete destruction" of Syria's chemical weapons, production facilities and "other relevant capabilities."
The United States and Russia are two of the five permanent Security Council members with a veto. The others are Britain, China and France.
"There is an agreement between Russia and the United States that non-compliance is going to be held accountable within the Security Council under Chapter 7," Kerry said. "What remedy is chosen is subject to the debate within the council, which is always true, but there's a commitment to impose measures."
Lavrov indicated that there would be limits to using such a resolution.
"Any violations of procedures <t40><t40>...<t$><t$> would be looked at by the Security Council and, if they are approved, the Security Council would take the required measures, concrete measures," Lavrov said. "Nothing is said about the use of force or about any automatic sanctions."
Kerry spoke of a commitment, in the event of Syrian noncompliance, to "impose measures commensurate with whatever is needed in terms of the accountability."
The agreement offers no specific penalties. Given that a thorough investigation of any allegation of noncompliance is required before any possible action, Moscow could drag out the process or veto measures it deems too harsh.
Kerry stressed that the United States believes the threat of force is necessary to back the diplomacy, and U.S. officials have said Obama retains the right to launch military strikes without U.N. approval, to protect American national security interests.
"I have no doubt that the combination of the threat of force and the willingness to pursue diplomacy helped to bring us to this moment," Kerry said.
However, a leading U.S. senator expressed concerns that without the threat of force, it's not clear "how Syrian compliance will be possible under the terms of any agreement."
Republican lawmaker Bob Corker of Tennessee said Syria's "willingness to follow through is very much an open question" and he did not want the negotiations to signal a "retreat from our broader national interests," including support for "moderate" opposition forces in Syria.
U.N. inspectors were preparing to submit their own report. Secretary-General Ban Ki-moon said Friday that he expected "an overwhelming report" that chemical weapons were indeed used on the outskirts of Damascus on Aug. 21.
A U.N. statement said Ban hopes the agreement will prevent further use of such weapons and "help pave the path for a political solution to stop the appalling suffering inflicted on the Syrian people."
Britain's foreign secretary, William Hague, said Saturday's development is "a significant step forward." Germany believes that, "if deeds now follow the words, the chances of a political solution will rise significantly," Foreign Minister Guido Westerwelle said.
The commander of the Free Syrian Army rebel group, Gen. Salim Idris, said in Turkey that the Russian initiative would "buy time" and that rebels will continue "fighting the regime and work for bringing it down."
He said if international inspectors go to Syria in order to inspect chemical weapons, "we will facilitate their passages, but there will be no cease-fire." The FSA will not block the work of U.N. inspectors, he said, and the "inspectors will not be subjected to rebel fire when they are in regime-controlled areas."
Idris said Kerry told him by telephone that "the alternative of military strikes is still on the table."