Russia warned late Tuesday that it would use a "military-technical" response to the US missile shield system which Washington wants to deploy in Poland and the Czech Republic.
Moscow has said the missile shield would increase tensions in Europe and expressed fears it could be used to spy on Russia instead of, as Washington says, protecting Europe against missile attacks by so-called rogue states like Iran.
The warning, reported by ITAR-TASS news agency quoting the Russian Foreign Ministry, came as US Secretary of State Condoleezza Rice completed a deal in Prague for the Czechs to host the system's radar installation.
"If the agreement with the United States liable for ratification by the Czech parliament eventually becomes a law, and the deployment of US missile defense elements really starts in direct proximity to our borders, we will have to use military-technical rather than diplomatic methods," the Russian ministry statement said.
"Russia will have to take appropriate measures and compensate the forming of potential threats to its national security. This is not our choice."
Washington played down Moscow's saber rattling, urging Russia to join its defense program as an "equal partner."
"We seek strategic cooperation on preventing missiles from rogue nations like Iran from threatening our friends and allies," said White House spokesman Gordon Johndroe. "We will continue to have dialog with the Russians."
Rice and her Czech counterpart Karel Schwarzenberg signed the treaty on Tuesday under which the Czech Republic is to host the radar base for the missile shield the US plans to build.
The US wants the radar base twinned with interceptor missiles in Poland, although negotiations with Warsaw have bogged down.
The signing in Prague went ahead even though the deal faces hurdles in the Czech parliament where the center-right government of Prime Minister Mirek Topolanek will likely need opposition votes to get it ratified.
But Schwarzenberg told reporters after the signing ceremony that he believed lawmakers would vote in favor of the program.
The US missile plan is not popular with the Czech public. An opinion poll last month showed that 68 percent of Czechs are against the project.
Police in Prague prevented about 1,000 protestors on Tuesday evening from marching to the Hrzansky palace to hand over a protest letter to Rice.
"We believe that this could start another arms race," Frantisek Smrcka, one of the protestors, told Reuters news agency.
The opponents want the government to delay further talks until after the November presidential election in the United States, in the hope that the new president will halt the project.
"Our government is unreasonably and madly rushing ahead," said radar opponent Jan Neoral, mayor of a village nearby the planned radar site.
Cold War rhetoric
Analysts said Moscow's threat is a strategy to increase European opposition to the shield.
Pavel Felgenhauer, a military analyst, said Moscow was using rhetoric reminiscent of the Cold War to discourage the Czech parliament from ratifying the shield agreement.
"That is why it is very unspecific, but sounds threatening," he told Reuters. "It is psychological pressure, the same sort that was used in the 1980s by the Soviet Union, when the United States deployed cruise missiles in Europe, in an attempt to boost the anti-missile, anti-US protests."
Russia's UN Ambassador Vitaly Churkin said the Foreign Ministry statement did not mean military action but rather a change in "strategic posture."
"If you are talking about military action, this of course is not the case," Churkin said in New York.