MOSCOW - President Obama and Russian President Dmitry Medvedev will try to break a deadlock in talks to replace a vital nuclear arms control treaty when they meet here today, with U.S. missile defense plans and Russian demands for sharper cuts in launchers presenting the key obstacles.

"Right now, there are very serious gaps in the Russian and American positions," said Sergei Rogov, director of the Institute for U.S. and Canadian Studies in Moscow, who has been monitoring the talks through Russian negotiators.

If the presidents emerge without the outline of a deal, it may be impossible to adopt an accord to replace the 1991 Strategic Arms Reduction Treaty before it expires in December, analysts say. That would unravel verification mechanisms that have been critical to reducing both countries' nuclear arsenals and could undermine global efforts against nuclear proliferation.

It would also cast a shadow over the three-day summit, as both leaders have made replacing the treaty, known as START I, the centerpiece of their efforts to "reset" bilateral relations badly strained during the Bush administration.

"If Obama leaves Moscow disillusioned, he may decide it's not worth the political capital, time, and energy to deal with so-called defiant rogues in Moscow," said Dmitri K. Simes, president of the Nixon Center in Washington. The Russians, meanwhile, are inclined to believe "that not much can be accomplished with the United States," he said.

Gary Samore, an Obama aide responsible for issues of weapons of mass destruction, said yesterday that he expected the presidents to announce progress on a treaty today but not a final deal.

"The negotiators have narrowed the differences, identified key issues, and I think it will be possible for the presidents to have a good discussion and, hopefully, reach agreement" on some issues, he said.

Negotiators have tentatively agreed on a modest reduction in the 1,700 to 2,200 deployed warheads permitted under the Moscow Treaty signed in 2002, perhaps to about 1,500.

But there has been no breakthrough in the stalemate over a U.S. plan to deploy missile defenses in Eastern Europe to counter a potential threat from Iran. Russia says the shield would undermine its ability to deter a U.S. nuclear strike, and Medvedev recently warned that Russia would not accept any treaty unless its concerns are addressed.

Meanwhile, U.S. officials have balked at Moscow's insistence on steep cuts in the numbers of long-range missile launchers and heavy bombers that each side can keep. START set a limit of 1,600 such "delivery systems" for each country, and U.S. negotiators have offered to reduce it to 1,000 to 1,100. But Russia wants the ceiling set closer to 600, Rogov said.

The Russian military is also worried about U.S. plans to refit missiles and bombers with conventional payloads, which it fears would extend U.S. military superiority and could be used to overwhelm Russia's nuclear forces. In data disclosed under START in January, the United States said it had about 1,200 delivery systems and Russia reported about 800.

The Obama administration can bring the U.S. count down by dismantling about 100 missile silos no longer in use. But achieving sharper cuts would mean taking down active missile silos, implementing costly modifications to submarines, and destroying bombers assigned to nonnuclear missions.

Pavel Podvig, an arms-control scholar at Stanford University, said none of these options is politically feasible because the administration has not completed a Congress-mandated review of U.S. nuclear strategy. If Obama agreed to any of them, Republican lawmakers, who generally oppose limits on delivery systems, "would eat him alive," Podvig said.

On missile defense, Obama has said he is still reviewing the plan inherited from President George W. Bush to deploy interceptors in Poland and a radar in the Czech Republic. The Obama administration has refused Russian demands to openly renounce the Polish-Czech plan.

Medvedev, who took office last year, is also under pressure not to back down on the missiles, a situation complicated by his partnership with his powerful patron and predecessor, Prime Minister Vladimir Putin.

In remarks broadcast on state television yesterday, Medvedev said he considered missile defense "interconnected" with the arms-control treaty. But he hinted at some flexibility, saying it would be "sufficient" for Obama to "show restraint and show an ability to compromise."

Obama, in an interview with Novaya Gazeta newspaper to be published today, repeated assurances that U.S. missile defenses were not aimed at Russia, saying: "Such thinking is simply a legacy of the Cold War. We have not yet decided how we will configure missile defense in Europe, but my sincere hope is that Russia will be a partner in that project."