The US' European missile defense shield is set to go live on Thursday, almost a decade after Washington proposed protecting NATO members from Iranian rockets and despite Russian warnings that the West is threatening peace in central Europe.

The shield will be declared operational by US and NATO officials at a remote air base in Deveselu, Romania, after years of planning, billions of dollars in investment and failed attempts to assuage Russian concerns that the shield could be used against Moscow amid high tension between Russia and the West.

"We now have the capability to protect NATO in Europe," said Robert Bell, a NATO-based envoy of US Defense Secretary Ash Carter. "The Iranians are increasing their capabilities, and we have to be ahead of that. The system is not aimed against Russia," he told reporters, adding that the system will soon be handed over to NATO command.

The US will also start construction on a second site in Poland on Friday that is due to be ready in 2018, giving NATO a permanent, around-the-clock shield in the alliance's northern reaches in addition to radar and ships already in the Mediterranean.

Russia is incensed at such a show of force in formerly communist-led Eastern Europe, where it once held sway. Moscow says the US-led alliance is trying to encircle it close to the strategically important Black Sea - home to a Russian naval fleet - where NATO is also considering increasing patrols.

The readying of the shield also comes as NATO prepares a new deterrent in Poland and the Baltic States following Russia's 2014 absorption of Crimea. In response, Russia's defense ministry recently announced that it is reinforcing its western and southern flanks with three new divisions.

Despite US assurances, the Kremlin says the missile shield's real aim is to neutralize Moscow's nuclear arsenal long enough for the United States to make a first strike on Russia in the event of war.

The shield relies on radars to detect a ballistic missile launch into space. Tracking sensors then measure the rocket's trajectory and intercept and destroy it in space before it re-enters the earth's atmosphere. The interceptors can be fired from ships or ground sites.

The Russian ambassador to Denmark warned a year ago that Danish warships would become targets for Russian nuclear missiles if Denmark were to join the shield project by installing radar for the system on its vessels. Denmark is upgrading at least one frigate to house a ballistic missile sensor.

Turkey already hosts a US radar, and the Netherlands has equipped some of its ships with radar. The US also has four ships in Spain as part of the defenses, while all NATO nations are contributing funding.