Russia will take "retaliatory steps" over a planned US missile shield in the Czech Republic, according to President Dmitry Medvedev.
"We are extremely upset by this situation," Mr Medvedev told a news conference at the G8 summit on the Japanese island of Hokkaido.
"We will not be hysterical about this but we will think of retaliatory steps," he said, while adding that he wants to continue talks on the issue with Washington.
The radar base will be set up after Secretary of State Condoleezza Rice signed an agreement in Prague on Tuesday.
The US plans to combine this with 10 interceptor missiles located in Poland, and wants the system to be in operation by 2012.
The US has branded Russia's "military means" threat over plans for a missile shield in Europe as "bellicose rhetoric".
The Pentagon said the comments were "designed to make Europeans nervous about participating" in the plans.
Moscow furiously opposes the plans, despite US assurances that it is designed to counter a threat from the Middle East, not from Russia.
In an earlier statement, the Russian Foreign Ministry statement said: "If a US strategic anti-missile shield starts to be deployed near our borders, we will be forced to react not in a diplomatic fashion but with military-technical means."
In February, then-President Vladimir Putin said Russia could deploy missiles in the Baltic Sea region of Kaliningrad, which borders Poland, if the US plans went ahead.
Moscow added that there was "no doubt that the grouping of elements of the strategic US arsenal faced towards Russian territory" would mean they had to "take adequate measures to compensate for the threats to its national security". It said: "This is not our choice."
A White House spokesman said the US planned to continue discussions with Russia.
He said: "We seek strategic co-operation on preventing missiles from rogue nations, like Iran, from threatening our friends and allies. He added that the US and Russia should be "equal partners".
The plans are unpopular with voters in the Czech Republic, while the US has so far failed to reach an agreement with Poland over the placing of missiles there.
On signing the deal with the Czech Republic, Ms Rice said the next American president will have to decide whether to continue with plans for the missile defence system.
She said: "It's hard for me to believe that that's not a capability an American president is going to want to have."
© Copyright of Telegraph Media Group Limited 2008.