Talks between the United States and Russia on a successor treaty to the Strategic Arms Reduction Treaty (START) will resume in mid-January next year, State Department spokesman P.J. Crowley said Tuesday.

Crowley said the U.S. delegation, led by Assistant Secretary Rose Gottemoeller, has returned from Geneva for a recess.

"Our goal remains to conclude a solid treaty for the president's signature as soon as possible, and we expect that the teams will resume their negotiations in Geneva in mid-January," Crowley said in a regular state department briefing.

The U.S. and Russian teams have been in intense talks in the past few months, aiming to clinch an agreement over a successor treaty to the START, which expired on Dec. 5. The eighth round of talks was held in Geneva on Nov. 9 - Dec. 19.

U.S. President Barack Obama said the sides are "quite close to an agreement" on further nuclear arms reduction after his latest meeting with his Russian counterpart Dmitry Medvedev on the sidelines of the UN climate talks in Copenhagen last week.

The Russian president said some technical details require "further work."

The negotiations have missed both Dec. 5 and the end of the year to produce a treaty. Crowley explained such negotiations take a significant amount of time.

"It's very, very complex," he said, "we've made progress..., we're in a pretty good position. But after a couple of months of very intensive work it was useful to take a break, and we'll resume this in January."

Steven Pifer, an expert in arms control in Brookings Institution, said in an interview with Xinhua that he thinks the negotiations have gone quiet well.

"First remember how complex this treaty is. They are basically negotiating in nine months a new treaty," he said, noting the START treaty itself took over six years to negotiate.

Russia's top military officer, Nikolai Makarov, said Monday that Russia and the United States were likely to clinch the new START treaty early next year.

Verification details

Crowley said the talks involve "complex issues" in terms of "numbers, verification and kinds of systems," while expressing confidence that the two sides will arrive at a satisfactory conclusion and agree on a new treaty that meets the national interests of both the United States and Russia.

Pifer, a former U.S. ambassador to Ukraine, said he thinks the remaining issue between the two sides is probably verification details.

He said the Russian side probably wants to reduce the amount of telemetry they share, which is the data a missile broadcasts during a test shot.

"With telemetry access, we'll have a good understanding of Russian missile performance, they'll have a good understanding of American missile performance, that kind of transparency is very reassuring," Pifer said.

The START, signed in 1991 between the United States and the Soviet Union, obliged both sides to reduce the number of their nuclear warheads to 6,000 and that of delivery vehicles to 1,600.

The new treaty's outline agreed to by Obama and Medvedev at a July summit in Moscow included slashing nuclear arsenals to 1,500 to 1,675 operational warheads and delivery vehicles to 500 to 1,000.

Pifer said the two sides now probably don't have much argument over the numbers. He said the number of operational warheads would probably stand at around 1,600, while that of delivery vehicles may stand between 700 to 800.

Also on Tuesday, Russian Foreign Minister Sergei Lavrov said the new treaty would envision "radical, unprecedented" slashes in strategic offensive weapons, and "interconnection between strategic offensive nuclear and non-nuclear weapons will be documented in the new treaty."

Pifer said the U.S. side wanted to mount conventional warheads on intercontinental missiles, such as the Trident Ballistic Missiles, replacing the nuclear warheads on them. The strategy is called "prompt global strike capability," giving the U.S. military the power to strike anywhere in the world in a 20 to 25 minute timeframe.

"I think the Russians really don't like the idea of these conventional warheads. But my sense is that the sides will probably go to this treaty and say if it's a warhead, whether it's nuclear or conventional, it counts in as a warhead," Pifer said.