MOSCOW — Prime Minister Vladimir V. Putin said Tuesday that the main obstacle to replacing the Strategic Arms Reduction Treaty, or Start, is Washington’s plan to build a missile defense system, which he said endangered the cold war-era balance of power.

“If we don’t develop a missile defense system, a danger arises for us that with an umbrella protecting our partners from offensive weapons, they will feel completely safe,” Mr. Putin told journalists during a working visit to Vladivostok. “The balance will be disrupted, and then they will do whatever they want, and aggressiveness will immediately arise both in real politics and economics.”

To preserve the balance, he said, Russia must develop new offensive weapons to counter the missile shield — or the United States must provide Russia with data on its missile defense plans in exchange for data on Russian weapons development.

It was not clear whether Mr. Putin’s comments, which were widely reported in the Russian news media, reflected a new Russian position or a negotiating tactic in the final stages of the Start talks, which American officials have said are close to a conclusion.

But Mr. Putin’s comments came three weeks after negotiators missed the Dec. 5 deadline for replacing Start, and appeared to reflect a lasting skepticism in Russia about American intentions, almost a year after Vice President Joseph R. Biden Jr. announced that it was time for the two countries to “press the reset button.”While President Obama has impressed Russian leaders, many question whether he is capable of changing Washington’s policies toward Russia, said Dmitri V. Trenin, director of the Carnegie Moscow Center.

“They would say that Obama is serious, he views the world differently, but the U.S. is a very big ship that cannot change its course dramatically in a few months,” Mr. Trenin said. “The people who see Russia as a problem are still there, and they can be found at the Pentagon. They also say Obama is here for eight years maximum, and he may not be able to withstand the pressures on him.”

The new treaty would require the countries to reduce their nuclear arsenals by at least one-quarter. Since the spring, Russian officials have sought to tie the Start follow-up talks to changes to the American missile defense plan, which they see as designed to counter their missiles. American officials say the shield is geared toward threats from Iran or North Korea, not Russia, and refused to include missile defense in the negotiations — a position that Ian C. Kelly, the State Department spokesman, reiterated in Washington on Tuesday in response to Mr. Putin’s remarks.

“While the U.S. has long agreed that there is a relationship between missile offense and defense, we believe the Start follow-on agreement is not the appropriate vehicle for addressing it,” he said.

In September, Mr. Obama announced that he would scrap plans for missile defense system sites in Poland and the Czech Republic, a central irritant to Russia, in favor of smaller ship-based interceptors that might later be positioned on land in Europe. The White House said the decision was a response to changing Iranian capacities rather than Russian complaints. But in any case, Moscow welcomed the move, with Mr. Putin calling the decision “correct and brave.”

Three months later, most people in the Russian government believe that Washington is advancing quickly toward a missile defense system, and that its goals have not changed, said Sergei A. Markov, a deputy with the governing United Russia party.

“Putin wants to stress exactly that this goal has not been dismissed,” Mr. Markov said.

A major tension is that the two countries are at different stages in their strategic military development: Russia is actively replacing and adding offensive weapons, and the follow-up treaty would require Russia to share information about the testing of new missiles. The United States, meanwhile, is under no obligation to share similar information about missile defense, said Mr. Trenin, meaning “the whole thing is asymmetrical.”

Mr. Putin’s comments left no doubt that he was playing a pivotal role in negotiations with Washington, a task that technically should fall to his protégé, President Dmitri A. Medvedev.

“It would be a huge obstacle in the talks if Putin now says we need limits on missile defense as part of this treaty,” said Steven Pifer, a Russia expert at the Brookings Institution. “It would be a huge setback, and it would make the treaty very hard, if not impossible, to conclude.”

At the moment, he said, Mr. Putin’s meaning “is not clear, and I suspect it won’t be clear until the negotiators sit down again in January.”

Mr. Markov said he was not surprised the talks had been difficult; from the first, at least for Russia, they have been freighted with outsize importance.

“It’s not just about the Start agreement, but about the status of the Russian Federation — whether Russia is a great power or not,” he said. “We have heard a lot from Washington that Russian interests should be limited to Russia’s borders. That means Russia is not recognized as a great power. And that’s why this negotiation is so difficult, because no one knows what Russia’s status is.”