Washington (CNN) -- The United States and Russia say they will be unable to reach a deal on an arms control treaty by the end of the year to replace their existing one, which expired this month.
Assistant Secretary of State P.J. Crowley told reporters Tuesday that the U.S. delegation -- led by Assistant Secretary Rose Gottemoeller -- had returned for a recess from the START negotiations in Geneva.
Talks are expected to resume in mid-January with new proposals.
"The team has gone through an intensive period of negotiations with their Russian counterparts over more than two months," Crowley said. "Our goal remains to conclude a solid treaty for the president's signature as soon as possible."
The 1991 Strategic Arms Reduction Treaty, commonly known as START, expired on December 5 but the leaders agreed to honor its spirit until a new treaty could be negotiated to replace it.
President Obama and Russian President Dmitry Medvedev, who met last week in Copenhagen, Denmark, had hoped to sign an agreement by the end of the year.
A statement on the Russian Foreign Ministry's Web site said, "Preparation of the new treaty is practically complete" but "a few problems have to be overcome in order to finalize the treaty and submit it to [the] presidents of Russia and the United States. This will be done very soon."
"Having resolved most issues discussed in recent months, delegations arrived at corresponding positions," the statement said. "The Russian side would like to emphasize the talks have been constructive and business-like. It reflects partnership and mutual respect which characterize the current stage in the relations between Russia and the United States."
Crowley also cited a few "hurdles" to an agreement, including the complexity of the weapons systems, agreement on the numbers of warhead reductions, and verification. But he said the two sides hope to finalize a deal to replace START when they meet next month.
"Clearly, over the course of these two months, we have made dramatic progress," he said. "There are still issues that we continue to work through, so there's still more work to be done. But I think we remain confident that, given good faith and the ongoing efforts of both sides, that this will get done."