After months of negotiations with Russia, Obama administration officials are hopeful about a breakthrough – possibly this week – that would enable the two sides to sign a successor to their most extensive nuclear weapons treaty before it expires Dec. 5.

The optimism stems from a trip to Moscow in late October by national security adviser James Jones, who gave his Kremlin counterpart a package of proposals for the new Strategic Arms Reduction Treaty, or START, according to U.S. and Russian officials. Moscow has not yet formally responded, but high-level Russian officials have reacted positively, senior U.S. officials said.

Russian President Dmitry Medvedev said in remarks released Saturday that both sides “have every chance to agree on a new treaty, determine new (weapons) levels and control measures and sign a legally binding document (by) the end of the year.” With U.S. policymakers and the Pentagon united behind Jones’s proposals, Kremlin policymakers have gone back to the Russian military to get its approval or perhaps recommendations for counterproposals.

Securing a replacement for the 1991 treaty is a critical first step in President Barack Obama’s ambitious global arms-control agenda. Analysts and lawmakers have watched nervously as the agreement’s deadline approaches, fearing a lapse in the complex verification procedures that are credited with providing stability between the nuclear giants. Both sides have discussed leaving those procedures in place until a new pact goes into effect.

U.S. officials’ optimism contrasted with concerns expressed recently by American and Russian analysts that the talks have not produced final agreement on key issues: limits on nuclear-capable launchers; verification procedures; U.S. proposals to put conventional warheads on strategic land- and submarine-based ballistic missiles, and missile defense systems. The United States remains reluctant to give much ground on a Russian request for strong language linking disarmament to missile defense.

The new START agreement will contain relatively modest cuts in the 1,700 to 2,200 deployed warheads allowed to each side under a June 2002 agreement between then-presidents George W. Bush and Vladimir Putin. At a summit in July, Obama and Medvedev agreed on a new ceiling of 1,500 to 1,675 for each side.

Although a new accord seems within reach by Dec. 5, it is still not likely to win ratification in the U.S. Senate for months. With that in mind, Sen. Richard Lugar, R-Ind., introduced a bill Thursday that would allow Obama to temporarily extend, on a reciprocal basis, privileges to Russian arms inspection teams that travel to the U.S.