WASHINGTON (CBSDC) — A security expert is claiming that tens of thousands of Russian soldiers rehearsed a military takeover of the Baltic Sea in March.

According to The Local SE, a report from Edward Lucas for the U.S.-based Center for European Policy Analysis claims that 33,000 Russian soldiers practiced seizing Gotland off Sweden’s east coast, Danish island Bornholm, Finland’s Aland islands and northern Norway.

“If carried out successfully, control of those territories would make it all but impossible for NATO allies to reinforce the Baltic states,” Lucas’ “The Coming Storm” report says.

Lucas says in his report that Sweden should intensify its cooperation with NATO.

“Edward Lucas wants to show that Northern Europe and Poland have the economic resources to defend themselves, but are far too disunited, but also that the Baltic Sea area is very important and that what happens here matters to the rest of Europe,” Johan Eellend, a security political analyst at the Swedish Defense Research Agency, said, according to The Local SE.

Peter Mattsson, a researcher at the Swedish Defency University, told The Local SE that the Baltic Sea is a “strategic spot.”

“A takeover of these islands would mean that NATO would not be able to send ships into the Baltic Sea and would make NATO irrelevant there,” he said. “It’s such a strategic spot.”

The Local SE reports that Lucas’ report came a week after Russia’s ambassador to Sweden warned the Scandinavian country that it would likely face military action if they joined NATO.

The alleged exercises took place from March 21 to March 25.