DEVESLU, Romania — A US anti-missile defense system in Romania aimed at protecting NATO members from threats by "rogue" nations became operational on Thursday in a move that infuriated Russia.

The missile interceptor station located in Deveselu, southern Romania, will help defend NATO members against the threat of short- and medium-range ballistic missiles, particularly from the Middle East, officials said.

"Today the United States and Romania make history in delivering this system to the NATO alliance," said US Navy Adm. Mark Ferguson, commander of Allied Joint Force Command Naples and US forces in Europe and Africa, at an inauguration ceremony alongside NATO chief Jens Stoltenberg.

But Russia sees the missile system as a security threat right on its doorstep, despite the US and NATO insisting it is not aimed at undermining Moscow's defenses.

"From the very beginning of this whole story, we have said that according to our experts' opinion, we are convinced that the deployment of the missile defense system is truly a threat to Russia's security," Kremlin spokesman Dmitry Peskov told reporters in Moscow.

Relations between NATO and Moscow have sharply deteriorated since Russia annexed Crimea from Ukraine in March 2014, sparking fears among other eastern European countries that they too could be the targets of Russian aggression.

Stoltenberg said the missile installation in Romania "represents a significant increase in the capability to defend European allies against proliferation of ballistic missiles" as it becomes part of a broader NATO missile shield with an installation in Poland as well. But he stressed that the system was not aimed at Russia and in fact was not capable of intercepting Russian missiles.

"The site in Romania as well as the one in Poland are not directed against Russia. The interceptors are too few and located too far south or too close to Russia to be able to intercept Russian intercontinental ballistic missiles," he said.

'Purely Defensive' 

The Deveselu site will host a battery of SM-2 missile interceptors and will officially be integrated into the NATO missile shield at the bloc's summit in Warsaw in July.

Work on the Deveselu site began in October 2013 and is thought to have cost $800 million.

The Western military alliance insists the role of the missile shield is a "purely defensive" response to external threats, notably from "rogue states," the term used to refer to Iran and North Korea.

The US ambassador to NATO, Douglas Lute, has described the activation of the missile system as a gesture of his country's commitment to Article 5 by which all 28 NATO members pledge a one-for-all, all-for-one response to any military threat if a member invokes the treaty clause in the face of an attack.

But Russia meanwhile is bolstering its forces to counter what defense officials said was the NATO buildup close to its borders.

"It is a step toward the military and political containment of Russia," senior foreign ministry official Andrei Kelin said of the deployment, Interfax reported.

Kelin warned that the system would "only worsen" the already tense relations between Russia and NATO.