While agreeing to work overtime to negotiate a replacement for the seminal 1991 Strategic Arms Reduction Treaty, or START, which expires at year's end, the two leaders vowed at the same time to jointly confront other perceived threats. They specifically mentioned the nuclear programs in Iran and North Korea and al-Qaida militants who have found refuge in Pakistan.
The statements could serve as a major boost for both Obama and Medvedev, who are new to the foreign policy proving grounds and are in need of the other's help.
If Medvedev is successful, with Obama, in midwifing the birth of a new nuclear reduction treaty, he will solidify his hold on Kremlin power, where former President Vladimir Putin is perpetually looking over the shoulder of his hand-picked successor.
Obama stands to gain a major ally in the foreign policy problems most vexing to his administration, particularly Iran, Afghanistan and North Korea.
Konstantin Kosachyov, the Kremlin-connected head of the parliament's foreign affairs committee, said the main outcome of the meeting was that it "broke the inertia of thinking that has accumulated on both sides."
"The meeting was needed to break that vicious circle, and it was absolutely successful," Kosachyov said, according to the RIA Novosti news agency.
Both men directed their negotiating teams to finish the task of setting broad outlines for a treaty to replace START by the end of July. That conceivably would leave time to get the new treaty approved in the U.S. Senate by the December expiration of the current agreement. But arms control experts say December is not a hard deadline so long as there is progress.
"The parties have gone from words to action," said Sergei Rogov, the head of USA and Canada Institute.
RIA Novosti reported that Rogov believed quickly reopening arms control talks "offers a real prospect that a new treaty will be signed before the year's end."
Rogov said that arms control talks will help create "favorable conditions for reaching agreements on a host of other issues, including the Iranian nuclear problem and cooperation in fighting the Taliban and al-Qaida."
Currently, the United States has 2,200 strategic nuclear warheads deployed; Russia has 2,800. Under the subsequent 2002 Treaty of Moscow, a plan negotiated under the Bush administration, the two sides committed to reducing their nuclear warheads to between 1,700 and 2,200.
But that treaty did not establish its own system for verifying compliance; instead it said verification would rest upon the existing provisions of the START treaty. But if START expires in December without a replacement in place, the Moscow Treaty would be left with no legally binding system for verification.
Obama has declared his belief that the United States and Russia should take the lead in ridding the world of nuclear weapons altogether. Russian and American arms control experts believe that the START replacement treaty would seek initially to cut strategic warhead arsenals to 1,500 on each side.
After their first meeting, the White House also announced that Obama was accepting Medvedev's invitation to visit Moscow this summer.
"Over the last several years, the relationship between our two countries has been allowed to drift," Obama told reporters after his meeting with Medvedev. "What I believe we've begun today is a very constructive dialogue that will allow us to work on issues of mutual interest."
Striking a similar tone with the U.S. president at his side, the Russian president said: "I am more optimistic of the successful development of our relations."
They did not immediately address the major issue of American preparations to deploy a missile shield in Poland and Czechoslovakia, but in their joint statement the United States acknowledged Russian concerns. Obama has said deployment of the anti-missile system still depends on its being proved an effective weapon.
Privately, his administration is believed ready to delay deployment for the sake of improving ties with the Kremlin.
Their newly-professed commitment to reinvigorate arms-control initiatives that have lain dormant for years caused a stir at the London site of a G-20 summit that seemed otherwise transfixed on a deepening worldwide recession.
Obama trumpeted the new arms undertaking as representing "great progress" between Moscow and Washington on areas where the two have mutual interests, although he also said he wouldn't try to minimize differences.
"What we're seeing today is the beginning of new progress in U.S.-Russian relations," he said.
White House spokesman Robert Gibbs noted that the two countries have not settled on a new cap for nuclear arms.
The session between the two presidents was not just a get-to-know-you meeting, said senior Obama administration officials who spoke on condition of anonymity to more freely describe the private discussions. Both sides worked for weeks ahead of time to develop two joint statements.
Obama and his aides were particularly pleased at what they saw as small progress on Russia's position on Iran, with Moscow coming closer to agreeing that Tehran could be pursuing nuclear weapons and thus pose a threat, and on agreement about the threat from extremists in Afghanistan and Pakistan.
But the talks were not all about agreement. Last August's devastating war between Russia and the former Soviet republic of Georgia came up, with Obama saying directly that Georgia's pro-Moscow separatist regions of South Ossetia and Abkhazia would never be recognized as independent by the United States, the officials said.
Obama and Medvedev made it clear that progress on a new arms-reduction deal must be made by the time of the U.S. president's planned visit to Moscow in July, the officials said.
Both sides recognize that negotiating a new treaty will be difficult, with many thorny issues to resolve.
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