Shocking satellite images have pointed to evidence of Russian nuclear build-up on the border with Europe.
New footage from CNN showed Russian military bases being rapidly upgraded and expanded in Kaliningrad, which sits on the border with Poland and Estonia.
The surveillance imagery reveals new nuclear bunkers, air bases and missiles in development at four different bases.
A US defence official called the upgrade “the biggest move we have seen” of Russia militarising its border with Europe.
The latest developments appear to have dramatically expanded in recent months, with significant work focusing on what is believed to be an active nuclear weapons storage site.
Admiral James Foggo III, commander of US Naval Forces Europe-Africa and the Allied Joint Force Command, told CNN: “If they want to challenge us, we will challenge them.
“We’re not going to be intimidated by those systems that are out there.
“It’s very important for the Russians because that is their port on the Baltic sea.”
One of the air bases involved is Chernyakhovsk, home to a Russian missile brigade, which has seen a new railway and improved aircraft landing system.
There are more than 40 new nuclear bunker sites, according to analysis from ImageSat International.
Hans Kristensen, the director of nuclear information at Federation of American Scientists (FAS), suggested that nuclear weapons could be moved there quickly in a crisis.
The satillite imagery also hints at “concealed activity taking place underneath” the bunkers.
NATO is set to carry out its largest ever military exercise close to another Russian border in Norway later this month
The massive NATO military exercise involving tens of thousands of troops from more than two dozen countries.
Speaking to international experts in Sochi this week, President Putin claimed that Russia would “destroy” its enemies in a nuclear war.
He warned Western powers that Russia is ready to take on anyone who uses a nuclear bomb against his country – but insisted he did not plan for a preemptive strike.