Kerry made clear that the U.S. believes the threat of force is necessary to back the diplomacy.
"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.
Under the deal, the U.S. and Russia are giving Syria just one week, until Sept. 21, to submit "a comprehensive listing, including names, types and quantities of its chemical weapons agents, types of munitions, and local and form of storage, production, and research and development facilities."
International inspectors, the U.S. and Russia agreed, should be on the ground in Syria by November and complete their initial work by the end of the month. They must be given "immediate and unfettered" access to inspect all sites.
Kerry said the two sides had come to agreement on the exact size of Syria's weapons stockpile, a sticking point.
U.S. officials, speaking on condition of anonymity because they were not authorized to publicly discuss details of the negotiations, said the U.S. and Russia agreed that Syria had roughly 1,000 metric tons of chemical weapons agents and precursors, including blister agents, such as sulfur and mustard gas and nerve agents like sarin.
These officials said the two sides did agree on the number of chemical weapons sites in Syria.
U.S. intelligence believes Syria 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, the officials said.
U.N. inspectors are preparing to submit their own poison gas report this weekend. 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.
Obama called for a limited military strike against Assad's forces in response, then deferred seeking congressional approval to consider the Russian proposal.
The commander of the Free Syrian Army rebel group, Gen. Salim Idris, told a news conference in Turkey that the Russian initiative was a "waste of time" and that rebels will continue "fighting the regime and work for bringing it down."
He said that if international inspectors come 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."