Sweden is reportedly preparing hundreds of nuclear war shelters to prepare for a potential attack from Russia amid growing concerns in the Baltics.

The Swedish Civil Contingencies Agency (MSB) has ordered a review of 350 civilian bunkers on the Baltic Sea island of Gotland, where Sweden has stationed permanent troops.

The shelters are designed to protect people against the shock wave and radiation from a nuclear detonation, as well as chemical and biological weapons.

Mats Berglund, head of civil protection at the MSB, told Sverige Radio the shelters should be checked by the end of the year.

Once a Cold-War era bunker and nuclear shelter, the Pionen White Mountain has been converted into a data centre and hosts some of WikiLeaks' servers (JONATHAN NACKSTRAND/AFP/Getty Images)

The island's 350 shelters have capacity for around 35,000 people, although Gotland has a population of nearly 60,000.

Over 65,000 shelters were established during the Cold War to protect the Swedish population from the potential threat of nuclear warfare.

All of the shelters are marked with a distinctive orange and blue logo, along with the word skyddsrum (shelter).

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It comes after the head of Sweden's intelligence agency warned there is "a real and serious threat against the security" of the country.

While Sapo head Anders Thornberg did not explicitly mention Russia, the country has been building up its military in response to Russia's annexation of Crimea and its "increasing pressure" in the region.

Non-Nato member Sweden has been upgrading its military with a sharp hike in spending and has urged local governments to prepare their civil defence infrastructure and procedures for a future war.

It recently reintroduced a military draft for both men and women over concerns about increased military action in the Baltic region.