A US missile defence site in Romania, aimed at protecting Europe from ballistic missile threats, has been activated.
It's housed in an old Soviet-built air base located 180 kilometres (110 miles) south-west of Bucharest, and will be part of a wider NATO defence shield.
The move has angered Russia, which opposes having the advanced military system in its former area of influence.
Forces TV's Kate Wathall spoke to Igor Sutyagin, a senior research fellow at the Royal United Services Institute who specialises in Russian studies, about the issue (see below).
NATO Secretary General Jens Stoltenberg tried to reassure the country as he spoke at an opening ceremony attended by US, NATO and Romanian officials.
He said the missile defence site "in no way undermines or weakens Russia's strategic nuclear deterrent", adding:
"This 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 added that the interceptors were designed "instead to tackle the potential threat posed by short and medium-range attacks from outside the Euro-Atlantic area".
US officials said the Romanian missile shield, which cost $800 million (£550 million), is intended to fend off missile threats from Iran and is not aimed at Russia.
NATO's Secretary General during his visit to the site
Mr Stoltenberg also noted that Moscow had unilaterally terminated co-operative dialogue about missile defence in 2013.
However, he said the alliance would continue to try and engage Russia in dialogue where possible. He said:
"In times of tension, keeping channels of communication open is even more important."
Russian president Vladimir Putin's spokesman, Dmitry Peskov, said Moscow was already taking measures for "securing the necessary level of security in Russia".
It comes as Russia prepares to trial a nuclear weapon that's reported to be able to destroy a whole country in seconds.
Click below, meanwhile, to see how the US defence shield would work...
Admiral Vladimir Komoyedov, chairman of the State Duma's defence committee, called the missile defence site a threat to Russia.
Mr Komoyedov, the former commander of the Russian Black Sea Fleet, told the Interfax news agency:
"This is a direct threat to us. They are moving to the firing line. This is not just 100; it's 200, 300, 1,000% aimed against us. This is not about Iran, but about Russia with its nuclear capabilities."
Romanian president Klaus Iohannis said his country wanted NATO to have a "permanent naval presence" in the Black Sea that respected international conventions, and called for increased security for NATO members in the south and east, which border Russia and the Middle East.
"It is important that a credible and predictable presence can be assured of the allied forces on the eastern flank, to balance the northern dimension with the southern and eastern flank," he said after meeting Mr Stoltenberg in the Romanian capital of Bucharest on Thursday.
On Friday, meanwhile, Polish and US officials will break ground at a planned missile defence site in the Polish village of Redzikowo, near the Baltic Sea.