London  - US President Barack Obama and Russian President Dmitry Medvedev moved to open a new chapter in relations between their two nations Wednesday by agreeing to draw up a new deal on nuclear disarmament.

At a meeting in London ahead of Thursday's summit of Group of 20 (G20) leading world economies, both presidents pledged to "move further along the path of reducing and limiting strategic offensive arms," in a joint statement issued after their first meeting since Obama's inauguration.

Seated side by side in armchairs, both Obama and Medvedev agreed that US-Russian relations had been marked by a sense of drift, and that the agreement to open talks on nuclear warheads represented a major breakthrough in ties between the two nations.

The US president also accepted an invitation to visit Moscow in July, to assess progress towards a new deal that would replace the US-Russian strategic weapons reduction programme (START) which expires at the end of 2009.

"My hope is that ... what we're seeing today is the beginning of new progress in the US-Russian relations," Obama said.

Under the START programme, in place since 1994, the two countries have already agreed to a strategic arsenal of no more than 6,000 warheads and 1,600 carrier missiles.

Wednesday's joint statement emphasized a commitment to build on START, which had "completely fulfilled its intended purpose," as well as further scaling back nuclear stockpiles held by Russia and the US.

The new deal would aim to "mutually enhance the security" of Russia and the US, according to the statement signed by both parties.

Obama praised the role of the Russian president in reaching such a quick agreement on the framework for a new treaty.

"I think that President Medvedev's leadership has been critical in allowing that progress to take place," Obama said after the leaders' discussion.

Medvedev in turn spoke hopefully of the follow-on meeting he is due to host later in the year.

"July is the warmest time in Russia and in Moscow, and I believe that will be exactly the feature of the talks and relations we are going to enjoy during that period in Moscow," the Russian president said.

Senior US administration officials said Obama had shown himself determined to "find ways to work on common interests," whilst also being "very candid where we disagree."

This included discussion on Georgia's breakaway regions, the official said, adding that, "the President also made clear the idea of a sphere of influence is an idea whose time is long past its due, not a 21st-century idea."

Bones of contention included America's key aim of preventing Iran from acquiring nuclear weapons. During the meeting, senior officials said, Russia and the US "came closer to having a mutual understanding of what that threat is."

Russia is the main supplier of technology and nuclear material which, Iran says, is purely for civil purposes.

Russia is also at odds with the US over Washington's plans to install a missile defence system in Poland and the Czech Republic, under a programme initiated by former president George W Bush.

Ahead of the meeting, Medvedev had repeated China's call to reevaluate the dollar as reserve currency. Obama, on the other hand, had rejected Chinese demands to replace the dollar with a global reserve currency.

"We should also think together of whether it might be expedient to introduce a world supranational reserve currency, potentially under the aegis of the International Monetary Fund," Medvedev told the Washington Post.

The US officials told Wednesday's press gathering that Obama's strategy towards Russia was to "develop an agenda based on interests."

Contrasting this to the US and Russian leaders' predecessors, George W Bush and Vladimir Putin, the official added, "The goal is not to have a personal relationship."

Relations between Bush and Putin soured towards the end of the outgoing US president's term of office, plummeting to a low point during Russia's military intervention in Georgia in August last year.

The US officials further described the steps which brought about Wednesday's agreements, starting with phone calls between Medvedev and Obama which led to an exchange of letters - personally delivered to Moscow, on part of the US - and followed up by a Geneva meeting between the countries' foreign ministers.

The result, officials said, was a "document of work ... not a document of principles or flowery language.

The US officials involved in the negotiations made it clear that there was still a lot of work ahead.

"The statement is the beginning of a long process," they said.

"And we have our eyes wide open about how difficult it will be in terms of getting real agreements." (dpa)