LONDON, April 1 -- President Obama and Russian President Dmitry Medvedev launched negotiations on a new nuclear arms treaty today, even as they agreed to pursue new and broader cooperation across a wide range of policy areas.

In a statement after a closed-door meeting, the two leaders pledged to begin work immediately on a new treaty on offensive nuclear weapons to replace the 2002 Moscow Treaty on Strategic Offensive Reductions, which expires at the end of this year. They committed to reducing their nuclear arsenals to levels lower that those mandated by the START treaty, which calls for both nations to have between 1,700 and 2,200 warheads by Dec. 31, 2012.

"The Presidents decided to begin bilateral intergovernmental negotiations to work out a new, comprehensive, legally binding agreement on reducing and limiting strategic offensive arms," the statement says. Obama told reporters that he will travel to Moscow in July, the date by which the two leaders said their negotiators should report progress on the new arms reduction treaty.

Obama, who like Medvedev is in London for the G-20 summit that begins with a dinner Wednesday night, also met Wednesday with British Prime Minister Gordon Brown and Chinese President Hu Jintao. The closed-door sessions focused on the global economic crisis, and Obama announced plans to visit China during the second half of this year.

At a joint news conference with Brown, Obama said the United States accepts its share of the blame for the economic problems that are spiraling around the globe but said recovery will depend on the world's nations acting together to spark growth and revamp banking and financial regulations.

"The United States does not intend to act alone, and we are not," Obama said.

After meeting for the first time, Obama and Medvedev also issued a broader statement that outlines new areas of planned cooperation while skirting some of the most contentious issues that have soured relations during the past several years.

Senior U.S. officials said the meeting turned around what had been a dramatically deteriorating relationship and set the two countries on a fundamentally new course. But they also characterized Obama as frank with his counterpart about American disagreements with Russia. They said he challenged Medvedev about his country's military incursion into Georgia last year and its claim of a broad sphere of influence over the countries of Eastern Europe.

In their joint statement, the two leaders pledged to cooperate on trying to rein in Iran's nuclear ambitions, contain the proliferation of nuclear technology and fight terrorism in Afghanistan and Pakistan. And they jointly expressed concern about North Korea's expected launch of a ballistic missile sometime soon.

"We, the leaders of Russia and the United States, are ready to move beyond Cold War mentalities and chart a fresh start in relations between our two countries," the statement reads. "In just a few months we have worked hard to establish a new tone in our relations. Now it is time to get down to business and translate our warm words into actual achievements of benefit to Russia, the United States, and all those around the world interested in peace and prosperity."

The statement reflects a desire by both nations to put more pressure on Iran to end its pursuit of nuclear weapons. While giving a nod toward Iran's claim that it is developing a civilian nuclear program, the document puts the Russians on the record calling on Iran to implement U.N. Security Council resolutions regarding the confirmation of its nuclear capabilities and intentions.

"Iran needs to restore confidence in its exclusively peaceful nature," the document says.

It acknowledges that "differences remain over the purposes of deployment of missile defense assets in Europe," but adds that the two leaders discussed the possibility of international cooperation in the missile defense field.

The statement does not attempt to resolve some of the stickiest issues that divide Washington and Moscow, in particular the disagreement over Russia's aggressive actions in the nation of Georgia and the deployment of missile defense equipment in Poland.

On Georgia, the statement says: "Although we disagree about the causes and sequence of the military actions of last August, we agreed that we must continue efforts toward a peaceful and lasting solution to the unstable situation today."

According to a senior U.S. official, Obama told Medvedev bluntly in their meeting that "we are not going to recognize South Ossetia and Abkhazia as independent states." Moscow recognized the two breakaway regions of Georgia as independent republics last year, but few other countries have followed suit. Obama "said very categorically, too, we don't recognize spheres of influence as a useful concept in the 21st century," the official said.

The launching of new arms-reduction talks is aimed at producing a treaty that contains aggressive techniques to verify warhead and missile levels, something that U.S. officials said the two countries have not attempted for almost 20 years since the START negotiations. They dismissed more recent treaties negotiated by President George W. Bush on the subject as "arms control lite" and said they have no intention of hosting frequent but brief meetings like those favored by Bush, calling them "drive-by summits" that produced little.

"This meeting in and of itself shows you we don't do drive-by summits; we have meetings of substance," one senior official told reporters. "Frankly, neither president, most certainly not our president, has the time to have 30 meetings."

On Iran, U.S. officials said Medvedev agreed to play a more substantial role than ever before in attempting to contain Iranian nuclear ambitions and to prevent the spread of nuclear technology around the globe.

The meeting between Obama and Medvedev lasted 70 minutes and began with a cordial exchange by the two men about their families and their shared background as lawyers, according to participants.

After the meeting, the pair spoke to reporters from a sitting area overlooking the rolling lawn of Obama's guest house. They were seated in arm chairs side by side, with a bowl of bright pink camellias between them and a bank of Russian and American flags behind them.

Senior U.S. officials described several weeks of intense negotiation leading up to the meeting. They said agreement on launching a new arms reduction treaty and many of the broader agreements were far from certain only three weeks ago.

The officials credited an earlier, private letter from Medvedev to Obama, and Obama's response to his Russian counterpart, for providing an opening to a broader statement today.

"Several weeks ago, the agenda was very narrow," said one top U.S. official. "Our president most certainly said to us, 'we don't want to have just a get-together meeting.' I get the sense that Medvedev seized the moment, too."

But the official said: "We are not being naive about this. The relationship with Russia is a complicated, big, difficult one."

Obama's meeting with Medvedev was followed a short time later by his first session with Hu, the Chinese president. Both meetings were held at Winfield House, the residence of the U.S. ambassador in London.

A joint statement from Obama and Hu after their session said the two nations were launching new strategic and economic dialogues as part of an effort to pursue closer relations. The strategic dialogue will be led by Secretary of State Hillary Rodham Clinton for the United States and Chinese State Councilor Dai Bingguo, the statement said. The economic effort will be led by Treasury Secretary Timothy F. Geithner and Chinese Vice Premier Wang Qishan.

The statement commits the United States and China to better military-to-military relations and to a resumption of consultation on nuclear nonproliferation and international security. Obama and Hu also pledged to work together to help rescue the world economy and reform the financial regulatory system.

At the news conference with Brown, Obama said the United States "has some accounting to do," because of a financial regulatory system that proved inadequate to oversee the growth of banks and hedge funds.

But he added that there has been an equal "mismatch between regulatory reforms and the highly integrated global markets" and said he expects a concerted approach to be announced at the G-20 summit.

"The world has become accustomed to the United States being a voracious consumer market and the engine that drives a lot of economic growth worldwide," Obama said. "If there's going to be renewed growth, it can't just be the United States as the engine. Everybody is going to have to pick up the pace."

Both Obama and Brown dismissed talk of dissension among the governments gathered here about how best to proceed with economic recovery and whether to spend more money to spark growth. Obama called reports of those disagreements "vastly overstated."

To reports that French President Nicholas Sarkozy might pull out of the summit if the negotiations don't go his way, Brown joked that he was "confident that he will still be sitting as we complete our dinner this evening."

Speaking before his meeting with Medvedev, Obama said he has no interest in "papering over" the "very real differences between the United States and Russia."

But he said that there are "broad interests" that can form the basis of a cooperative relationship, and that he detects "great potential" for working together on a broad range of topics.

"Both the United States and Russia and other nuclear powers will be in a much stronger position to strengthen what has become a somewhat fragile, threadbare nonproliferation treaty if we are leading by example," Obama said.

As Obama began his string of meetings -- in addition to Brown, Hu and Medvedev, Wednesday's lineup included British opposition leader David Cameron -- thousands of protesters lined the streets in anticipation of the economic summit. Besides demonstrations on economic conditions and climate change, large antiwar protests were planned by the Stop the War Coalition, the Campaign for Nuclear Disarmament, the Palestine Solidarity Campaign and other groups.

On a lighter note, Obama declined during the news conference with Brown to be drawn into predictions about England's World Cup qualifying match Wednesday night, citing controversy in the United States over the teams he picked his NCAA tournament bracket.

"The last thing I'm going to do is wade into European football. . . . I didn't get a briefing on that, but I sense that would be a mistake," he said to laughter, undoubtedly scoring points with Europeans for not using the American term "soccer" to describe the game.

The new U.S. president was also careful to meet the protocol requirements that go with carefully scripted overseas trips.

In the first moments at the microphone, Obama mentioned the "special relationship" that the United States has with Britain, a pet phrase that his spokesman failed to use several weeks ago, prompting mocking headlines in the British tabloid press.

Obama also made certain to articulate his anticipation about the audience he and first lady Michelle Obama will have with Queen Elizabeth II later Wednesday.

"As you might imagine, Michelle has really been thinking that through," Obama said. "In the imagination of people throughout America, what the queen stands for and her decency and her civility and what she stands for, that's very important."

Correspondent Mary Jordan contributed to this report.

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