Pushing his vision of a nuclear weapons-free world, President Barack Obama returned to Prague on Thursday to sign a pivotal treaty aimed at sharply paring U.S. and Russian arsenals _ and repairing soured relations between the nations.

On a sunny morning, an upbeat Obama was in the Czech capital after an overnight flight from Washington. His main mission: seal the hard-fought arms reduction deal with Russian President Dmitry Medvedev and signal progress to a watching world.

Obama has not been able to deliver on all of the nuclear aspirations he outlined in Prague just a year ago _ visions on disarmament that helped earn him the Nobel Peace Prize. Back then, a landmark speech to an enthusiastic crowd gathered in a Prague square included a promise to quickly seek ratification of a comprehensive nuclear test ban.

That goal remains unfulfilled. But on Thursday, the Czech capital will serve as the venue of a pledge achieved.

At noon in the Spanish Hall _ a lavish Renaissance chamber within the Czech's capital's ornate presidential castle complex _ the U.S. and Russian presidents will sign the "New START" treaty.

With that, they will commit their nations to slash the number of strategic nuclear warheads by one-third and more than halve the number of missiles, submarines and bombers carrying them, pending ratification by their legislatures.

Before the signing, Obama and Medvedev began private talks. They offered no initial comments to the press but rather two smiling photos: with their host at Prague Castle, Czech President Vaclav Klaus, on steps overlooking a picturesque view of the sun-drenched cityscape below; and a second one in a gilded, ornately decorated castle chamber.

The new treaty will shrink those warheads to 1,550 over seven years. That still allows for mutual destruction several times over. But it will send a strong signal that Russia and the U.S., which between them own more than 90 percent of the world's nuclear weapons, are serious about disarmament.

The treaty would then need to be ratified by the U.S. Senate and Russia's legislature.

It opens the way for further cuts. U.S. Secretary of State Hillary Rodham Clinton said Tuesday that Obama has instructed his national security team to pursue another round of arms reduction talks with Russia, to follow up on the replacement for the 1991 Strategic Arms Reduction Treaty being signed Thursday.

The aim would be to conduct wider talks to include for the first time short-range U.S. and Russian nuclear weapons as well as weapons held in reserve or in storage.

Beyond deeply reducing nuclear arsenals, the U.S. sees "New START" as a key part of efforts to "reset" ties with Russia, which had become badly strained under the Bush administration and engage Moscow more in dealing with global challenges. Among these is Iran's defiance of U.N. Security Council demands that it curb its nuclear program to ease fears it seeks to make nuclear arms.

The new pact is only part of the Obama administration's new nuclear strategy. It will be signed only days after the White House announced a fundamental shift that calls the spread of atomic weapons to terrorists or rogue states _ nations, like North Korea or Iran that do not abide by nonproliferation rules _ a worse menace than the Cold War threat of mutual destruction.

"For the first time, preventing nuclear proliferation and nuclear terrorism is now at the top of America's nuclear agenda," Obama said Tuesday in detailing key components of an in-depth nuclear strategy review.

While the formal occasion is signing the treaty, Obama is expected to use the occasion to push for Russian support for new U.N. sanctions to punish Iran's refusal to give up uranium enrichment. Chances of enlisting China, an even more stubborn opponent of Iran sanctions than Moscow, improve if the Kremlin is on board.

Other U.S. nuclear initiatives will follow the Prague signing, with the White House planning to lead calls for disarmament in May at the United Nations during an international conference on strengthening the Nuclear Nonproliferation Treaty. Before that, government leaders from more than 40 countries will gather in Washington next week to discuss boosting defenses against terrorists seeking nuclear weapons.

The deep cuts in warheads and delivery systems and a legally binding system to ensure against cheating, makes "New START" the most significant nuclear arms treaty in a generation, and Medvedev, in comments published before the signing lauded it as "an important step" in disarmament and arms control efforts.

Some Russian arms control analysts say Russia badly needed the deal to ease the burden of replacing a large number of Soviet-built missiles which need to be decommissioned for age. "This treaty is in Russia's best interests," said Sergei Rogov, the head of the USA and Canada Institute, an influential think tank that advises the Kremlin on foreign policy.

Russian Foreign Minister Sergei Lavrov said the deal could "open new possibilities for developing the U.S.-Russian partnership" _ but warned Moscow reserves the right to withdraw if the planned U.S. missile defense system grows into a threat.

Russia has welcomed Obama's decision to scrap the previous administration's plans for missile defense sites in Poland and the Czech Republic while voicing concern about the prospects of a revamped U.S. project, including a planned facility in Romania.

Lavrov said that the site in Romania poses no immediate threat, but added that Russia will closely watch further U.S. missile deployments and could opt out of the new treaty if it sees that U.S. missile interceptors acquire a capability to intercept Russia's strategic missiles.

But Brian McKeon, a senior adviser to Vice President Joe Biden, said Washington believes that the defense system does not affect strategic stability.

"The President has made clear that he is committed to continuing to develop and deploy that system," said McKeon, writing in the White House blog.

McKeon, a staff member of the National Security Council, has been charged with heading up the ratification effort for the treaty.

Senators were consulted throughout negotiations, leading to language that ensures investments in a reliable deterrent stockpile and places no constraints on missile defense development. But the signing of the treaty would start the first formal briefings on the deal in the Senate by the administration negotiating team, White House press secretary Robert Gibbs said.

Lobbying for bipartisan support at home, Gibbs told reporters traveling with Obama: "We are hopeful that reducing the nuclear threat remains a priority for both parties."

Russia shares Obama's goal of a nuclear-free world, but other nations must join the disarmament process, he said.

Moscow also warned that the U.S. plans to fit some of its nuclear-tipped missiles with conventional warheads could be an obstacle to ridding the world of nuclear weapons.

Lavrov argued that Russia views such weapons as just as destabilizing as nukes.

Russia hasn't tried to follow the U.S. example partly because most of its missiles lack precision for hitting pinpoint targets with conventional explosives. Its conventional military forces has steadily degraded, prompting the military to rely increasingly on nuclear weapons.

While the docile Russian parliament will do the Kremlin's bidding on the treaty, the ratification process in the U.S. Senate could be troublesome. Fearing potential trouble, Moscow has said that Russian lawmakers will synchronize their moves to ratify the deal with the U.S. legislators.

Sensitive to East European concerns, Obama is tending to other business while in Prague _ hosting a dinner for leaders from 11 Central and Eastern European nations formerly in or near Moscow's orbit, who worry about the Kremlin's post-communist push for influence.

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AP White House Correspondent Jennifer Loven contributed to this story from Prague.

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On the Net:

http://www.whitehouse.gov/blog/2010/04/07/a-new-start