US and Romanian governments are expected to make a formal announcement on Friday, declaring that the so-called Aegis Ashore site is "technically capable," officials said.
"That means all the major components of the missile defense system, including the missiles, are in place, and have been handed over to military commanders," an official told Reuters.
Military personnel must still integrate the site with NATO's broader ballistic missile defense system before it is ready for combat, the official said, adding that it should be compliant early next year.
Navy spokesman Lieutenant Commander Tim Hawkins stated that the site was "an important step in our efforts to protect against the growing threat posed by the proliferation of ballistic missiles of increasingly greater ranges, lethality and sophistication."
Friday's expected announcement comes after a year of planning and work by the US Missile Defense Agency.
At the same time, US officials are deliberating how to respond to an October 10 Iranian ballistic missile launch, an event argued to be in possible violation of a UN Security Council resolution.
The Iranian missile test does not violate the terms of the landmark nuclear deal reached this summer between Iran and world powers, but it does have US lawmakers questioning whether Tehran will honor the international agreement.
In July, Iran agreed to curb its nuclear program in exchange for the lifting of most sanctions.
Riki Ellison, founder of the nonprofit Missile Defense Advocacy Alliance, said last week's successful test of the Aegis Ashore system off the shore of Hawaii was the final step preceding Friday's announcement.
"This system is now in place to protect southern Europe from any specific threat from Iran should they decide to continue to break the treaty,” Ellison was quoted as saying by Reuters.
