Russia and the United States on Monday began a fresh round of talks on renewing a key Cold war-era nuclear arms reduction treaty that expires at the end of the year.

Negotiators began the first day of their meeting on the Strategic Arms Reduction Treaty (START) behind closed doors at the Russian mission in the western Swiss city of Geneva, a Russian diplomat said.

"The Americans arrived as expected at 11.00 am (0900 GMT) at the Russian mission to start the second round."

The talks move to the US mission on Tuesday and might not continue for a scheduled third day if the negotiators from Washington and Moscow manage to complete their agenda early, she added.

Officials from both sides expect to reveal few details after the scheduled meeting, which sets the scene for a US-Russian summit on July 6 to 8.

First results from the START negotiations, which began in Moscow two weeks ago, are due to be unveiled by presidents Barack Obama and Dmitry Medvedev when they meet in the Russian capital, a Russian diplomat said.

US State Department spokesman Ian Kelly said in Washington: "On the details of the negotiation, I think we prefer to keep that in private right now. I think that's just the best way to conduct these kinds of negotiations."

START, signed in 1991 just before the break-up of the Soviet Union, which bound both sides to deep cuts in their nuclear arsenals, is due to expire on December 5.

The agreement this year to seek its renewal marked the first tangible step in the thaw in US-Russian relations heralded by the Obama administration.

But, on top of the complex technical issues involved in the landmark disarmament treaty, the negotiations are dogged by bargaining over US plans for an anti-missile defence shield partly stationed in Europe, a project which has angered Russia.

US Secretary of State Hillary Clinton handed Russian Foreign Minister Sergei Lavrov a mock 'reset' button in Geneva when they met for the first time in March and hatched the work plan to renew START.

But Lavrov last month also reiterated Moscow's desire to take account of the situation surrounding the planned US anti-missile shield in Europe in the START talks.

US officials said that they "intend to discuss the full range of issues around missile defence but not in the context of the START talks."

Obama has ordered a review of planned extensions into Europe of the defence shield, which the United States has insisted is meant to counter an Iranian threat.

But Russia regards the shield, which would partly be based in former eastern bloc countries close to its borders, as a threat to its own security.

"The sides are not in the same position. Obama needs a result to demonstrate that the 'reset' of US-Russian relations is getting somewhere," said Evgeny Volk of the US-based Heritage Foundation.

Veteran disarmament expert Jozef Goldblat suggested an elegant way out: a five-year extension of START that would give time for negotiations on a "broader" disarmament deal between the world's biggest nuclear powers.

But the talks underway now could also allow Washington to generate a climate "propitious" for help on other issues where it needs Moscow's support, such as Iran and North Korea's controversial nuclear programmes, said Goldblat who is based at the Geneva International Peace Research institute.