Russia's foreign ministry says US activation of a European missile defence shield in Romania, scheduled for Thursday, violates a treaty on nuclear forces.
A ministry statement quoted by Interfax news agency said it was a breach of the Intermediate-Range Nuclear Forces (INF) Treaty, signed in 1987.
The US says the Aegis system is a shield to protect Nato from long-range missiles and is no threat to Russia.
Romania is hosting part of the system.
A ceremony will be held at a Nato airfield in Deveselu, southern Romania, on Thursday to mark the start of Aegis operations there.
"This decision is harmful and mistaken, because it is capable of upsetting strategic stability," said Mikhail Ulyanov, head of the Russian foreign ministry's department for proliferation and arms control issues.
Nato and US officials say the system has been developed to track and intercept missiles fired from a "rogue" state. In the past Iran was mentioned in that context, but the US has also had North Korea in mind.
For years the US has been testing the Aegis system on warships too.
An incoming missile would be destroyed in space, before it could re-enter the Earth's atmosphere.
On Friday another phase of the project will be launched in Poland, with a groundbreaking ceremony at Redzikowo, near the Baltic Sea. Aegis missiles are to become operational there in 2018.
Mr Ulyanov said Russia's interests "are being affected in a direct way by this".
He said the Americans' MK-41 launch system could also be used to fire cruise missiles, not just air defence missiles.
"From our viewpoint this is a violation of the Intermediate-Range Nuclear Forces Treaty," he said.