Obama had wanted a new deal in place before the end of the year, but that appeared unlikely.
Emerging from private talks with Russian President Dmitry Medvedev on the sidelines of the U.N. climate conference, Obama expressed confidence that a successor pact will be agreed to in a "timely fashion." Medvedev said technical details still needed to be worked out.
Both leaders made only brief statements to reporters and took no questions. Neither one said anything about a possible timetable for signing a deal.
"We've been making excellent progress," Obama said. "We are quite close to an agreement. And I'm confident that it will be completed in a timely fashion."
Medvedev largely echoed Obama's expressions of optimism over a potential deal.
"Our positions are very close," the Russian leader said.
Three Russian news agencies quoted Medvedev aide Sergei Prikhodko as saying a treaty signing will not happen this year. Prikhodko said no signing date was being announced to avoid putting excess pressure on the negotiators, who are meeting in Geneva.
U.S. officials, speaking on condition of anonymity because of the sensitivity of the talks, said negotiations had become hung up on a disagreement over how to monitor the development of new intercontinental ballistic missiles.
Officials said U.S. negotiators would continue working with their Russian counterparts through the weekend. But negotiators plan to break for the Christmas holiday and return to the bargaining table in the new year, a U.S. official said Friday, speaking on condition of anonymity because he was not authorized to speak on the record.
The new deal would replace the 1991 Strategic Arms Reduction Treaty, or START I, that expired Dec. 5.
Signed by Soviet President Mikhail Gorbachev and U.S. President George H.W. Bush, that treaty required each country to cut its nuclear warheads by at least one-fourth, to about 6,000, and to implement procedures for verifying that each side was sticking to the agreement.
At a summit in Moscow last July, Obama and Medvedev agreed to cut the number of nuclear warheads on each side to between 1,500 and 1,675 within seven years, as part of a broad new treaty. They initially had instructed negotiators to seek a fully ratified deal by the Dec. 5 expiration.