Poland and the United States reached an agreement Thursday to base American missile interceptors in Poland in exchange for augmenting Polish forces with U.S. Patriot missiles.

"We have crossed the Rubicon," Poland's Prime Minister Donald Tusk said on television. The deal will see 10 missile defence interceptors located on Polish soil.

Tusk said the agreement, initialed by negotiators late Thursday in Warsaw, includes a "mutual commitment" between the two nations — beyond that of NATO — to come to each other's aid in case of danger.

The deal comes after more than 18 months of negotiations. It still needs approval from the Polish government and the signature of U.S. Secretary of State Condoleezza Rice at a date to be specified.

Referring to the "mutual commitment" part of the agreement, Tusk said that NATO would be too slow in coming to Poland's defence 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," Tusk said. "Poland wants to be in alliances where assistance comes in the very first hours of — knock on wood — any possible conflict.

"This is a step toward real security for Poland in the future."

Poland and other former Soviet satellites and republics in eastern Europe have been unsettled by Russia's powerful military incursion into Georgia in recent days.

The U.S. has also reached an agreement with the Czech Republic to place a radar component of the missile defence system in that country. The deal still needs approval from the Czech parliament.

The White House hailed the agreement with Poland.

"We believe that missile defence is a substantial contribution to NATO's collective security," presidential spokeswoman Dana Perino said. She said it was not an attempt to antagonize Russia, which opposes the system.

But the chairman of Russia's parliamentary foreign affairs committee, 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.

With files from the Associated Press