American and Polish officials stuck firmly by their deal, signed Thursday, for Poland to host a system that Washington says is meant to block missile attacks by rogue nations like Iran.
Moscow is convinced the base is aimed at Russia's missile force, however, and the deal comes as relations already are strained over the fighting between Russia and U.S.-allied Georgia over the separatist Georgian region of South Ossetia.
"Poland, by deploying (the system) is exposing itself to a strike - 100 percent," Gen. Anatoly Nogovitsyn, the deputy chief of staff of Russia's armed forces, was quoted by the Interfax news agency as saying.
He noted Russia's military doctrine sanctions the use of nuclear weapons "against the allies of countries having nuclear weapons if they in some way help them." Nogovitsyn said that would include elements of any strategic deterrence system, according to Interfax.
Russian President Dmitry Medvedev said the deal "absolutely clearly demonstrates what we had said earlier - the deployment has the Russian Federation as its target."
However, speaking at news conference with German Chancellor Angela Merkel, Medvedev appeared to take a softer position than Nogovitsyn's on the Polish base, saying that "it is sad news for all who live on this densely populated continent, but it is not dramatic."
Condoleeza Rice responded to the Russian threat from the Georgian capital, accusing the Russians of refusing to accept U.S. assurances that the missile defense system is not aimed at them and not their business, reports CBS News Chief Foreign Correspondent Lara Logan.
"Poland is an independent country. And it's an ally of the United States. And it's a democratic country, to whose security the United States is committed" through NATO, Secretary of State Condoleezza Rice said during a visit to Georgia's capital, Tbilisi.
"Russia should welcome having democracies on its border, not threaten them," she said.
Polish Foreign Minister Radek Sikorski said Poland was willing to let Russia inspect the future missile base to give Moscow "tangible proof" that it is not directed against Russia's arsenal, the Polish news agency PAP reported.
At a news conference earlier in the day, Nogovitsyn reiterated Russia's frequently stated warning that placing missile interceptors in Poland and a linked radar post in the Czech Republic would bring an unspecified military response. But the general's subsequent reported statement substantially stepped up the war of words.
Russian troops went deep into Georgia in the fighting, raising wide concerns that Russia could be seeking to occupy parts of a small, pro-U.S. neighbor that has vigorously lobbied to join NATO, or perhaps force the Georgian government to collapse.
"I think the Russian behavior over the last several days is generally concerning not only to the United States but to all of our European allies," Pentagon spokesman Bryan Whitman said when asked about Russian threats against Poland.
He also suggested that earlier U.S. offers for broad cooperation with Moscow on the missile defense program might be reevaluated considering the latest developments.
The missile deal, struck Thursday after more than 18 months of talks, must still be approved by Poland's parliament and signed by Rice during a future visit to Warsaw, possibly next week.
Polish Prime Minister Donald Tusk said the United States agreed to help augment Poland's defenses with Patriot missiles in exchange for placing 10 missile defense interceptors in the eastern European country.
He said the deal also includes a "mutual commitment" between the two nations to come to each other's assistance "in case of trouble." That clause appeared to be a direct reference to Russia, whose resurgent muscle worries Polish leaders.
"Simply, the existence of this installation increases Poland's security," Polish President Lech Kaczynski said.
Making a visit to Russia, German Chancellor Angela Merkel suggested Russia be included in the missile defense system.
"From my point of view, this deal is not directed against Russia," she said in Sochi at a joint news conference with Medvedev. "We will continue to advocate that talks be continued on how we can convince Russia of this and also ... include it," she added.