"The most serious threat we see right now is war preparations," Säpo chief counter-intelligence analyst Wilhelm Unge said at a press conference on Monday.
While stressing that such preparations did not necessarily mean anything dramatic, he said: "It's no secret that Russia is engaged in this. It's a little bit worrying."
Unge said Russia's intensified interest in Sweden was evidenced by simulated flight attacks on Swedish targets as well as attempts to recruit spies, increased signals intelligence, and the purchase of a significant number of maps.
"If you weren't carrying out war preparations against Sweden, you probably wouldn't have any military intelligence here. The intelligence service is part of the Russian general staff and just the fact that they are here indicates some kind of intent," he said.
The simulated flight attacks were a particular point of concern for Säpo.
"You don't carry out these kinds of things unless you can actually conceive carrying out an attack in the future," Unge added, refusing to go into further details.
The news came as part of Säpo's annual intelligence assessment, which also pointed the finger at 14 other countries with espionage interests in Sweden including China and Iran. Russia, however, was revealed to have the strongest presence in Sweden.
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"Russia is the biggest intelligence agent in Sweden. They're interested in everything really - politics, finance, technology, and military information," Unge explained.
"It's one of few countries that has a very broad intelligence interest in Sweden."
Unge added that the intensified spying was likely a result of Russia's involvement with Ukraine and Crimea, but the analyst remained tight-lipped about further details.