WARSAW, Poland — Poland and America reached an agreement today to base American missile interceptors at Poland, going ahead with a plan that has angered Russia and threatened to escalate tensions with the region’s communist-era master.

Prime Minister Donald Tusk said America had agreed to help augment Poland’s defenses with Patriot missiles in exchange for placing 10 missile defense interceptors in the eastern European country.

“We have crossed the Rubicon,” he said on the TVN24 news channel, referring to America’s consent to meet Poland’s demands.

Mr. Tusk said the agreement was initialed by negotiators late today at Warsaw and includes a “mutual commitment” between the two nations — beyond that of NATO — to come to each other’s assistance in case of danger.

That was an obvious reference to the force and ferocity with which Russia rolled into Georgia in recent days, taking the key city of Gori and apparently burning and destroying Georgian military outposts and airfields.

The agreement still needs approval from Poland’s government and a final signing from Secretary of State Rice at a yet unspecified date.

The White House celebrated the agreement. “We believe that missile defense is a susbstantial contribution to NATO’s collective security,” a presidential spokeswoman, Dana Perino, said.

She said it was not an attempt to antagonize Russia, which opposes the missile defense system.

A Russian lawmaker and parliamentary foreign affairs committe chairman, Konstantin Kosachev, warned that the deal would worsen tensions between Moscow and Washington, which already are being strained by Russia’s military offensive in Georgia.

While Washington says the defense system is meant to guard Europe against missile-armed states like Iran, the Kremlin feels it is aimed at Russia’s missile force, and Kosachev told the Interfax news agency the deal will spark “a real rise in tensions in Russian-American relations.”

America has also reached an agreement with the Czech Republic’s government to place a radar component of the missile defense system in that country. That deal still needs approval from the Czech parliament.

Talking about the “mutual commitment” part of the agreement, Mr. Tusk said that the North Atlantic Treaty Organization would be too slow in coming to Poland’s defense if Poland were 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 — 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,” Mr. Tusk said.

“This is a step toward real security for Poland in the future,” he added.

Poland and other former Soviet satellites and republics in eastern Europe have been unsettled by Russia’s poweverful military incursion into Georgia.

The deal was reached after more than 18 months of back-and-forth, often terse, negotiations.

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Associated Press writer Foster Klug in Washington contributed to this report.