WARSAW, Poland - The United States has entered a decisive phase in a plan to set up missile defense sites in Eastern Europe - a system Washington says is aimed at protecting itself and its allies against potential attacks from the Middle East.

But the prospect of sophisticated US radar and interceptor systems in formerly communist Eastern Europe has led Russian military leaders to warn of a new arms race. The system "would create a clear threat for Russia," Col. Gen. Vladimir Popovkin, the chief of Russia's Space Forces, warned Monday.

The United States told Polish leaders it wants to open formal negotiations on the possibility of locating ground-based interceptor missiles in their country as part of a larger missile defense system, a US Embassy spokesman in Warsaw said Monday.

Poland's Defense Minister Radek Sikorski indicated a willingness to talk with "our most important ally" on the issue, but said nothing had been decided.

The request comes after two years of exploratory talks and after the neighboring Czech Republic's prime minister said Saturday that Washington had asked to base a radar station in his country as another part of the system.

Washington has repeatedly sought to reassure Russia it has nothing to fear from the system - a message Andrew Schilling, a spokesman for the US Embassy in Warsaw, echoed Monday. He told The Associated Press a US missile defense system in Europe would be solely aimed at countering "the evolving Middle Eastern ballistic missile threat."

Some experts single out Iran as the motivator behind the US push to develop the system, meant to act as a protective umbrella over most of continental Europe with sensitive radars able to detect ballistic missiles and interceptors that could shoot them down.

"This is completely driven by the threat from Iran," said Riki Ellison, president of the Missile Defense Advocacy Alliance, a non-governmental group that promotes missile defense in the belief it increases world security.

"As Iranians continue to grow both their nuclear capabilities and their ballistic missile capabilities, this site in central Europe is needed," Ellison argued. "Not only for Europe and the troops that the United States has stationed in Europe ... but also for secondary protection of the United States."

Schilling said the request to Polish officials was first made Friday and a "formal diplomatic note" would follow. The US hopes talks on the plan will begin "in the course of the next several months," he added.

Agreement is not guaranteed. Both the Czech and Polish parliaments would have to first approve the plan. Although their governments expressed their support, they are weak and face broad public opposition to the plan. Many fear it will raise the risk of attacks on their countries.

"The current governments are in favor because both of them are right-of-center and Euro-skeptic and they think that as far as security is concerned, they have to maintain very close ties with the United States," said Jiri Pehe, a Czech political analyst and director of New York University in Prague.

By supporting US missile defense, the two European Union members are showing they do not trust the EU to guarantee their security - and are perhaps even trying to prevent the formation of a unified EU defense policy, Pehe said.

So far, four countries have working missile defense shields, including the US, which has interceptor bases in California and Alaska. Russia, Israel and Japan also have systems, while about 15 countries are also working to develop them, Ellison said.

In past months, Russian military leaders have occasionally threatened that an expansion of the US system so close to its borders could provoke a new arms race.

But Pehe, the Czech analyst, said Russian threats should be taken with a grain of salt.

"Russians in the past have threatened to do all kinds of things - for example when NATO was enlarged," Pehe said. "I think they are aware that this particular system, if it's ever built, isn't aimed against them. I would see this more as pressure - and a bit of bluffing."