RTS1139V
Swedish soldiers attend a rehearsal in front of the Royal Palace in Stockholm, Sweden June 18, 2010. Sweden has reinstated the military draft and investigated infiltration by spies in response to Russian aggression. Reuters

Sweden's intelligence agency investigated several cases of foreign spies trying to infiltrate government agencies in 2016 amid "a real and serious threat against the security" of the country, government officials said Thursday.

The head of Swedish intelligence agency SAPO, Anders Thornberg, told reporters at a news conference they'd never faced a "bigger and more complex task." The group released its annual report on the nation's security Thursday.

Read: Is Sweden Safe? Military Draft Enacted Amid Russian Aggression, Security Fears

"There's an ongoing intelligence gathering in our country, we keep seeing how people are being approached, we're seeing electronic attacks ... all available means are being used," Thornberg said.

RTS114BN
Swedish armored personnel carriers are seen in Visby harbor, island of Gotland, Sweden Sept. 14, 2016. Reuters

While Thornberg declined to give specifics about how many foreign spies SAPO had investigated, or what state actors they were affiliated with, he did describe Russia as the largest threat to the Scandinavian nation, which has historically remained neutral in military conflicts. The report comes amid growing tensions with Russia, which has a long history of using hacking and cyber attacks against countries in Europe.

Sweden, which is not a member of NATO, reinstated the military draft earlier this month after abandoning mandatory conscription in 2010. At the time of the announcement, a defense ministry spokesperson admitted that "Russian military activity" was one of the reasons for the reinstatement of the draft.

Late last year, Sweden's Civil Contingencies Agency, or MSB, sent letters to local governments urging municipal authorities to prepare their communities and infrastructure "in terms of war." The move was part of the country's return to its "totalförsvarsplaneringen" defense strategy, which calls for the deployment of economic and civilian entities and resources to defend against foreign aggression.

"This strategy is not new," said the MSB's Magnus Dyberg-Ek. "We used it during the Cold War, and we are going to now strengthen coordination regarding civil defense."