Negotiations with Russia to replace an expired Cold War-era arms control treaty have bogged down and appear unlikely to be concluded by the end of the year as the White House had hoped.

As the two sides seek a breakthrough, U.S. President Barack Obama and his Russian counterpart, Dmitry Medvedev, plan to discuss the nuclear negotiations in a meeting Friday on the sidelines of United Nations climate talks in Copenhagen, Denmark. The two leaders are not expected to seal a deal.

U.S. officials, speaking on condition of anonymity because of the sensitivity of the talks, say negotiations with Russia to replace the Strategic Arms Reduction Treaty have become hung up on a disagreement about how to monitor the development of new intercontinental ballistic missiles.

Obama and Medvedev initially had instructed negotiators to seek a fully ratified deal by the Dec. 5 expiration of START. Recently Obama had expressed hopes that a deal could be completed by the end of this year.

The Obama administration has sought to make the negotiations a vehicle for demonstrating improved relations with Russia. They hope that greater cooperation on arms control can lead to Russian help on stickier issues including efforts to rein in Iran’s suspected nuclear ambitions.

Officials said U.S. negotiators would continue working with their Russian counterparts on the treaty through the weekend in Geneva after the meeting of the two presidents. Top negotiators may pause for the Christmas holidays, however.

On Thursday, U.S. Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov discussed disagreements standing in the way of a deal.

Lavrov blamed the U.S. delegation for slowing negotiations in the past few days. He told reporters in Moscow that the talks have now resumed their pace, but a deal is unlikely to be reached in time for Obama and Medvedev to sign it when they attend the climate summit in Copenhagen on Friday. He urged the United States to accept deeper cuts and less intrusive verification measures.

U.S. officials said Russian negotiators were seeking changes from the original treaty on the encryption of missile flight data. The now-expired treaty banned such encryption so that each side could monitor missile tests from a distance. Using such data, monitors could determine whether the other side was developing missiles restricted by the treaty.

According to Daryl Kimball, executive director of the Washington-based Arms Control Association, Russia has less of an interest in monitoring such data because it is seeking to upgrade its missile arsenal while the United States has not been testing new missiles.

“The missiles the U.S. has are the most accurate, deadly ones in existence,” he said.

Despite the disagreements, U.S. and Russian officials continue to express optimism that the treaty can be concluded soon. White House spokesman Robert Gibbs said Thursday that the United States will not bend for a quick deal.

“It doesn’t make sense to get something just for the sake of getting it, if it doesn’t work for both sides,” he said.

The expired START pact, signed by Soviet President Mikhail Gorbachev and U.S. President George H.W. Bush, 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.

Obama and Medvedev agreed at a Moscow summit in July to cut the number of nuclear warheads that each possesses to between 1,500 and 1,675 within seven years as part of a broad new treaty.

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