Russia seems to have upgraded a nuclear weapons storage bunker in its Kaliningrad enclave, according to a new report.

The Federation of American Scientists (FAS) on Monday published satellite images purporting to show a storage facility being deepened and covered with a new concrete roof.

“It has all the fingerprints of typical Russian nuclear weapons storage sites,” Hans Kristensen, the director of the nuclear information project at FAS, told the Guardian.

“There is a heavy duty external perimeter of multilayered fencing. The bunkers themselves have triple fencing around them as well. These are typical features from all the other nuclear weapons storage sites that we know about in Russia.”

Work on the bunker reportedly began in 2016 and the new roof was added in the summer.

“Its a site we have been monitoring for quite some time and there have been and there have been some upgrades in the past but nothing as dramatic as this one. This is the first time we’ve seen one of the nuclear bunkers being excavated and apparently renovated,” Kristensen said.

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An image from a report from the Federation of American Scientists on the upgrade of a nuclear weapons storage facility in Kaliningrad. (Digital Globe via Getty Images)

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Although the images don’t prove that there are actually nuclear weapons in Kaliningrad now, they do show that the site is active.

It’s unclear, according to the FAS, whether the Russian military already has warheads there or plans to locate some there in the future.

Kaliningrad, which is one of the venues for the World Cup, is seen by analysts as part of Vladimir Putin’s efforts to push against NATO expansion in eastern Europe, according to the Guardian.

In January the Russian military said it had built the necessary infrastructure to accommodate mobile Iskander-M missiles, which are capable of carrying nuclear warheads to a range of 310 miles.

Kaliningrad also serves as a base for Russia’s Baltic fleet.