It comes as part of a renewed phase of Russian military build-up in the area that initially began in March this year, researchers say.

Sky News spoke to analysts about the open-source evidence available around two of the sites and asked what it can tell us about the situation on the border.

Among them are Buk air defence systems. The weapon gained notoriety after it was used by Russian separatists in the Donbas region of eastern Ukraine to shoot down a passenger flight in 2014, killing all on board.

A video, posted last week, was filmed at a station used to unload equipment for the Pogonovo training ground, just south of Voronezh.

It's one of a series of videos circulating online showing assets believed to belong to the 41st Combined Arms Army, usually based in central Russia, headed to the site.

"The unit appears to be carrying live ammunition - we saw several trucks carrying cylinders that match those used to carry missiles for the air defence platforms we identified," global intelligence company Janes told Sky News.

Whether or not these air defence systems would be used at this camp is unclear, Janes added.

But it is unusual for a significant quantity of missiles to be at a site like Pogonovo. And the presence of both the defence systems and their artillery indicates an "army-level" deployment at the camp, according to Janes.

The Pogonovo camp is one of the primary locations in the recent stage of the build-up.

Military hardware began being transported from the central part of Russia into the border region with Ukraine in March and April this year.

Equipment was moved there during this early stage, before some was transported north to another camp at Yelnya in the latest phase of the military build-up, which began at the end of October.

It's estimated there could now be as many as 92,000 Russian troops at camps across the border area.

While larger camps like Pogonovo and Yelnya are well documented, analysts have also identified activity at a number of smaller camps in the border region.

Satellite imagery from the end of November obtained by one researcher shows a camp just over 60 miles from the border, near the city of Kursk.

Since then, videos showing assets being transported by train in the Kursk region have circulated online.

"From what we’ve seen so far it appears we are looking at the build-up of another combined arms army camp from an army not previously identified as taking part in this phase of the build-up, the 6th Combined Arms Army," Janes told Sky News.

The unit usually operates over 700 miles away, in the Leningrad Oblast.

The camp near Kursk is just one of the smaller sites picked up by open-source researchers in the recent weeks. The emergence of such camps is a phenomenon that was not observed in the March/April phase of the build-up.

"Smaller camps are harder to detect and can increase in size slowly, which matches with what we’ve seen when units are deploying - slowly and often at night," said Janes.

"This differs significantly from the spring build-up, when units were being deployed at a much higher frequency, in larger numbers, and during all hours."

While the camp near Kursk does not yet have army-level deployment like Pogonovo, it could do in the future. But Janes say it's too early to tell.

The Data and Forensics team is a multi-skilled unit dedicated to providing transparent journalism from Sky News. We gather, analyse and visualise data to tell data-driven stories. We combine traditional reporting skills with advanced analysis of satellite images, social media and other open source information. Through multimedia storytelling we aim to better explain the world while also showing how our journalism is done.

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