In a sign that a new arms deal with Russia may be close, US President Barack Obama is sending two top national security aides to Moscow to work on clearing the last obstacles.National Security Adviser James Jones was heading to the Russian capital Wednesday, and Joint Chiefs Chairman Adm. Mike Mullen was also going, White House spokesman Mike Hammer said. He said the trip is "primarily to discuss the remaining issues left to conclude a new START treaty."The 1991 treaty expired last month. But Obama and Russian President Dmitry Medvedev agreed last July to seek a new one slashing the arsenals of both sides to between 1,500 and 1,675 warheads each.Talks in Geneva broke for the holidays in December amid lingering differences over verification and missile defense. The talks are set to resume on Monday. Last week, Undersecretary of State for Arms Control Ellen Tauscher told reporters the two sides are "really close to an accord."US negotiators have resisted Russia's demand to include in the deal a plan for monitoring US missile defense interceptors being deployed in Europe. But Moscow has also been loath to grant US experts access to Russia's data on new missile tests.A senior official couldn't say if progress had been made on either point. The official, speaking on condition of anonymity because of the sensitivity of the nuclear talks, said it was unclear if the trip by Jones and Mullen "could do the trick."Meantime, US efforts to line up fresh sanctions against Iran were also expected to come up in Moscow. Out of patience with Teheran's defiance of UN calls for it to stop enriching uranium, the administration has been pressing Russia and other key allies to agree to a package of additional punitive steps.The administration has been seeking the new START accord as a major benefit from its "reset" of relations with Moscow.But encryption of missile telemetry has proven one of the toughest issues to resolve. Current rules forbid encryption, so experts can freely monitor instructions sent from missile ground controllers. "The Russians were pushing to remove that," said James Collins, a former US ambassador to Moscow who's now at the Carnegie Endowment for International Peace."I would be pretty optimistic we're going to get it signed in the next number of weeks," Collins said. "The vast majority of the work is done."