Talks on a new US-Russian nuclear disarmament treaty are going better than initially expected, a top Russian diplomat told the RIA-Novosti news agency on Wednesday.

"The degree of progress is beyond the expectations that existed when we started," Deputy Foreign Minister Sergei Ryabkov was quoted as saying.

The United States and Russia have been seeking to agree a successor to the 1991 Strategic Arms Reduction Treaty (START), a cornerstone of Cold War-era nuclear arms control, before it expires on December 5.

US President Barack Obama is due to visit Moscow next week for meetings with his Russian counterpart Dmitry Medvedev at which START is expected to be a central issue.

Obama and Medvedev had a "detailed discussion" by telephone about their upcoming summit, the Kremlin said in a statement posted on its website late Tuesday evening.

"In particular, the presidents placed significant emphasis on the topic of reducing strategic nuclear weapons. They discussed various aspects of the nuclear weapons issue in light of the positions that have been reached by the two countries' negotiating teams," it said.

"The two leaders agreed to instruct their negotiators to intensify efforts in order to reach concrete results.

"Both sides expressed their confidence that the upcoming summit will provide great momentum and create a more productive atmosphere for bilateral relations, and allow the presidents to get to know one another better."

START led to huge reductions in the US and Russian atomic arsenals after its signing in 1991.

But negotiations to find a successor treaty have been held up by a dispute over controversial US plans to place missile-defence facilities in eastern Europe, which Russia opposes.