The newly activated US missile defences in Romania are a direct threat to Russia's international security and stability and the country is considering its response, the Foreign Ministry in Moscow said.

"We still regard the destructive activity of the United States and its allies in the sphere of missile defence as a direct threat to international and regional security and stability," ministry spokesperson Maria Zakharova was quoted as saying by TASS on May 12, following the commissioning ceremony attended by senior US officials in Deveselu in Romania. "The strategic situation in Europe is becoming much more complicated because of this."

Russia has long cited risks for international security and strategic stability posed by the unrestricted unilateral deployment of the US strategic missile defences, Zakharova added. It also offered different options of settling the missile defence issue and was willing to cooperate closely on the issue, "up to the joint establishment with Nato of an antimissile architecture in Europe, which would be built in line with the principle of sectors". However, Nato was not interested in the proposal, she added.

Other Russian officials assumed a similar tone. "The declarative assurances that the US and Nato missile defense system is not targeted against Russia cannot convince us," Alexander Grushko, Russia's permanent representative to Nato was quoted as saying. "We can see [the statements'] worth from the unwillingness of the US and Nato to in any way take into account the achieved international agreements on the Iranian nuclear programme during the allegedly 'adaptive' alignment of the missile defence system. Notably, the so-called ‘Iranian nuclear threat’ was presented earlier as the main incentive for its creation," Grushko said.

Meanwhile, the US side denies that the missile shield is aimed at Russia, saying it is rather a reaction to threats from the Middle East.

"As long as Iran continues to develop and deploy ballistic missiles, the United States will work with our allies and partners to defend NATO and its allies against this threat," Deputy Defense Secretary Bob Work said on May 12 at the Deveselu site, located on a former Romanian air base.

"This site, nor the site in Poland, has [no] capability - none whatsoever - to undermine Russia's strategic deterrent," Work said. "It is a defensive system. It is fully compliant with existing arms control regimes."

Russian officials have repeatedly classed the deployment of Mk 41 missile launchers in Romania a violation of the 1987 INF Treaty, because the Mk 41 is a "dual purpose launcher" and US ship-based systems of this type are capable of launching both interceptor missiles and Tomahawk medium-range cruise missiles.

Experts have cited various ways in which Russia could potentially respond to Nato's move.

"Russia can respond by stationing of Iskander missiles in Kaliningrad region, which will target Poland and Romania," Kirill Koktysh, a lecturer at the Moscow international relations institute (MGIMO), was quoted as saying by Sputnik news agency.

According to Konstantin Sivkov, president of the Academy of Geopolitical Issues in Moscow, Russia should respond by developing new-generation super-powerful nuclear missiles.

"A response should be asymmetrical," he was quoted as saying by RIA Novosti. "[Russia should reply to] the deployment of the US missile defence by developing means for guaranteed destruction of the US."

"At this point, it could only be achieved by developing weapons that could cause geophysical changes on the territory of the United States," he went on to say. "I am talking about nuclear missiles of super-high calibre that, for instance, could trigger eruption of the Yellowstone Caldera [supervolcano in the norther US] or a strong tsunami from explosions deep in the ocean."

Meanwhile, Russian legislators suggested that Russia could send military submarines to the US shore.

"There is a need to look for technical shenanigans and opportunities for going round the missile defence or, better, to keep submarine containment forces near US shores," Vladimir Komoyedov, head of the defence committee of the State Duma lower house of parliament, was quoted as saying by Interfax.

Tensions between Russia and Nato have been building since Russia's annexation of Crimea in March 2014, but there has been a marked escalation in recent weeks. In April, a flyover by Russian jet fightersclose to the guided missile destroyer USS Donald Cook in the Baltic Sea caused an angry reaction from Nato.

The incident overshadowed Nato and Russia's first direct talks in two years which had been aimed at  easing tensions and addressing key mutual security issues but only hardened the atmosphere of confrontation. Russia said earlier in May that it will form three new military divisions to face off with Nato troops due to be deployed in Poland and the Baltic states.