U.S. President Barack Obama and his Russian counterpart Dmitry Medvedev struck an optimistic tone even as they conceded that they were unlikely to sign a deal this year on a successor to an expired nuclear arms control treaty, as they had hoped.

The two leaders met on Friday as negotiators are seeking to bridge differences on elusive details of a successor to the Strategic Arms Reduction Treaty.

Mr. Obama said on Friday that they were “quite close.” He had wanted a new deal in place before the end of the year, but that appeared unlikely.

The hold up has denied the White House a quick boost in its efforts to demonstrate improved relations with Moscow. The Obama administration had identified a successor to the START treaty as among the most achievable areas of cooperation with Russia, as it seeks broader help from Moscow on issues including reining in Iran’s suspected nuclear ambitions.

Although the 1991 START treaty expired Dec. 5, both countries have agreed to continue to honour its main provisions, pending the completion and legal ratification of a successor treaty.

Emerging from private talks with Mr. Medvedev on the sidelines of the U.N. climate conference, Mr. Obama expressed confidence that a successor pact will be agreed to in a “timely fashion.”

Mr. 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,” Mr. Obama said. “I’m confident that it will be completed in a timely fashion.”

Mr. Medvedev largely echoed Mr. Obama’s expressions of optimism.

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, Mr. Obama and Mr. 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.

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