WARSAW, Poland - Poland and the United States struck a deal Thursday that will strengthen military ties and put an American missile interceptor base in Poland, a plan that has infuriated Moscow and sparked fears in Europe of a new arms race.

"We have crossed the Rubicon," Polish Prime Minister Donald Tusk said, referring to U.S. consent to Poland's demands after more than 18 months of negotiations.

Washington says the planned system, which is not yet operational, is needed to protect the U.S. and Europe from possible attacks by missile-armed "rogue states" like Iran.

The Kremlin dismisses that claim, saying that Washington is really attempting to undermine Russia's nuclear deterrent.

U.S. officials also said the timing of the deal was not meant to antagonize Russians leaders at a time when relations already are strained over the recent fighting between Russia and Georgia over the South Ossetia region.

In an interview on news channel TVN24, Tusk said the United States agreed to help augment Poland's defences with Patriot missiles in exchange for placing 10 missile defence interceptors in the eastern European country.

He said the deal also includes a "mutual commitment" between the two countries to come to each other's assistance "in case of trouble."

That clause appeared to be a direct reference to Russia, which has threatened to aim its nuclear-armed missiles at Poland -- a former Soviet satellite -- if it hosts the U.S. site.

Poland has all along been guided by fears of a newly resurgent Russia, an anxiety that has intensified with Russia's offensive in Georgia, a former Soviet republic. The incursion, along with the bombing of military posts and airfields inside Georgia, has underlined a palpable fear in the region of Russia's renewed vigour and confidence.

In past days, Polish leaders said the war justified Poland's demands that it get additional security guarantees from Washington in exchange for allowing the anti-missile base on its soil.

Talking about the "mutual commitment" part of the agreement, Tusk said that the North Atlantic Treaty Organization would be too slow in coming to Poland's defence if threatened and that the bloc would take "days, weeks to start that machinery."

"Poland and the Poles do not want to be in alliances in which assistance comes at some point later," Tusk said. "It is no good when assistance comes to dead people. Poland wants to be in alliances where assistance comes in the very first hours of -- knock on wood -- any possible conflict."

He said that armed with Patriot missiles, Poles "will be able to effectively protect our territory."

But after the deal was announced, both American and Polish officials sought to play down any connection to the Georgian war.

"This is not linked to the situation in Georgia," the chief U.S. negotiator, John Rood, told The Associated Press, after the pact was initialed. "We had made these arrangements for this round of negotiations before the conflict in Georgia, and so we just merely continued with the schedule we had."

In Washington, White House spokeswoman Dana Perino also said the timing was not meant to tweak Russia. "We believe that missile defence is a substantial contribution to NATO's collective security," she said.

In initial reaction from Russia, the parliamentary foreign affairs committee chairman, Konstantin Kosachev, was quoted by Interfax news agency as saying the agreement will spark "a real rise in tensions in Russian-American relations."

After Tusk announced the deal, it went through an initial signing ceremony late Thursday in Warsaw, but still needs approval from Poland's government and parliament and a final signing by U.S. Secretary of State Condoleezza Rice at a yet unspecified date.

At the signing, Polish Foreign Minister Radek Sikorski said the deal would strengthen the U.S., Poland and NATO.

Earlier this year, NATO endorsed the U.S. plan to expand its global missile defence shield with the planned site in Poland and a linked radar tracking base in the Czech Republic.

"Only evil people should be afraid of our agreement," Sikorski told reporters after Rood and his Polish counterpart, Andrzej Kremer, initialled the agreement at the Foreign Ministry.

The U.S. earlier reached an agreement with officials in the Czech Republic to place the radar component of the missile defence shield in that country. That deal still needs approval from the Czech parliament.