The Nordic country has 64,000 defence bunkers -- more than nearly any other nation in the world -- with space overall for around seven million people.

Since Sweden joined NATO in March 2024, its Civil Contingencies Agency (MSB) has stepped up inspections of the shelters, some of which are large enough to accommodate thousands of people at a time.

According to the MSB, civil defence shelters provide protection against shockwaves and bomb fragments, the blast and heatwaves from a nuclear weapon, radioactive fallout, gas from chemical weapons and biological weapons.

The government is also investing in improving the emergency services' capacity to operate during conflicts, strengthening cybersecurity and replenishing medicine stocks.

MSB said on Monday it had begun a huge project to modernise the nuclear shelters, a task it expected to take "two to three years".

So far, work has started on 25 of the 80 especially large shelters, it said.

In 2025, MSB plans to replace filters which help protect people in the shelters from chemical and radiological weapons.

The latter spread radioactive contamination into the environment.

Prime Minister Ulf Kristersson said in January Sweden was "not at war but there is not peace either", citing hybrid attacks, suspected sabotage in the Baltic Sea and a proxy conflict fought on its soil.

Stockholm slashed military spending after the end of the Cold War but began increasing it again when Russia unilaterally annexed Crimea in 2014.

The authorities reactivated Sweden's "total defence" strategy -- combining both military and civil defence activities -- in 2015.

They began strengthening it further after Russia's full-scale invasion of Ukraine in 2022.

Among other things, the government has created the post of minister for civil defence to work alongside the armed forces minister, so civilians can be mobilised as well as the military.