A preliminary deal has been signed by Poland with the US, concerning plans to host part of the controversial US defence shield.
The proposal says that in exchange for help strengthening Polish air defences, the US will base 10 missile interceptors in Poland.
According to the US, the system will protect itself and Europe against long range missile attacks by “rogue states”.
As Russia has condemned the plans, the deal is expected to heighten tension between the US and Russia, say correspondents.
In Europe, the military balance would be upset by the project, said Moscow, who warned that it would have no choice but to point its own missiles at the installations.
Because of Russia’s involvement in the conflict in Georgia, relations between Washington and Moscow are currently strained.
The agreement is bound to anger the Russians, who vehemently oppose the system, say the BBC’s Adam Easton, in Warsaw.
After Russia’s involvement in Georgia, Washington has come round to their way of thinking, said Polish officials.
Russia is seen by Poland as a bigger threat to its security than so-called “rogue states” such as Iran, but it is different in the US, add correspondent.
Shortly after details of the deal with the US were made public, Russian foreign minister Sergei Lavrov was reported to have cancelled a scheduled visit to Poland.
The agreement has been announced on national television by Donald Tusk, Polish prime minister, shortly before it was signed by Deputy foreign minister Andrzej Kremer and US chief negotiator John Rood.
In exchange for hosting the 10 interceptor missiles in a former military base near Poland’s Baltic Sea coast, Washington had agreed to meet Warsaw’s main demands, said Mr Tusk.
He added that in return the US has agreed to help modernise the Polish armed forces and locate Patriot missiles and a garrison of US servicemen in Poland to beef up its air defences.
After Moscow threatened to target its missiles at any eventual bases, Poland is reported to have demanded the extra security help as part of the deal.
In July, an agreement has been signed by the US with the Czech Republic, to base tracking radars there, as part of the missile defence system.
The US wants the sites to be in operation by about 2012.