"This update, which has been taking place across the Aegis Ballistic Missile Defence system fleet, did not provide any offensive capability to the Aegis Ashore missile defence system", the statement read.
A US Terminal High Altitude Area Defence (THAAD) system was deployed to the Deveselu base for the time of the upgrade and will now be redeployed as planned.
NATO stressed the upgrade was part of the US European Phased Adaptive Approach to ballistic missile defence, announced in 2009. The Aegis Ashore station will track potential threats outside the bloc.
Russia has been concerned about the potential ability of US missile systems in Europe to fire Tomahawk long-range subsonic cruise missiles, banned under their bilateral treaty. On 2 August, the United States officially withdrew from the Intermediate-Range Nuclear Forces (INF) Treaty, six months after announcing its decision to suspend obligations under the treaty over Russia's alleged violations of the accord. Russia has denied violating the INF Treaty and has pointed out to the United States’ moves in Europe in breach of the accord.
Russian President Vladimir Putin also said Washington bears full responsibility for possible consequences of this move and that Russia never violated the treaty.