Secretary of State Condoleezza Rice insisted Wednesday that a planned US missile shield in eastern Europe is defensive, just before she was to sign a deal to station part of the system in Poland.

Rice's remarks after meeting Polish President Lech Kaczynski were a renewed attempt to persuade Russia that the system - 10 missile interceptors in Poland and a tracking radar in Czech Republic - is not aimed against Moscow's strategic nuclear arsenal.

Polish Foreign Minister Radoslaw Sikorski and Rice were signing a deal Wednesday that sets up the US interceptor base in Poland in return for US help in bolstering Polish air defences.

Rice said the missile shield is "purely defensive" and strengthens strategic cooperation with Poland, a former Soviet-bloc nation that was among the first to join NATO after the Cold War ended.

The timing of last week's accord has fuelled speculation that Russia's invasion of Georgia spurred Poland to close the deal after some 18 months of hard bargaining. US and Polish diplomats have denied a link.

Russia has fiercely opposed the missile shield, due to be up and running by 2013, saying it would respond by targeting its two former satellites.

Rice repeated US assurances that the system was not aimed against Moscow.

"We're talking about a missile defense system that couldn't possibly be aimed at the Russian nuclear deterrent," she told the BBC on Tuesday. "Russia has thousands of nuclear warheads. This is for small missile attacks of the kind that Iran might launch."

In July, the Czech government agreed to host the system's radar in a military area near the German border.

Approval by the Czech and Polish parliaments is required before the project - the first US military bases in the two former Warsaw Pact nations - can go ahead.

Kaczynski called the signing an "important day" in Polish history during a televised address to the nation Tuesday night.

Poland had taken a step to "strengthen its position in the world" by sealing an agreement with Washington, he said.

Poland agreed last Thursday to host part of the US missile shield in return for military aid that includes a battery of Patriot air defence missiles.

The US plan has been unpopular in both countries, but a recent poll showed Polish opinion had swung in favor of the shield in the wake of the fighting over South Ossetia between Russia and Georgia.

Many Poles view Russia's military incursion into Georgia as proof that Poland needs a strong defence.