"I don't rule out NATO membership in any way", Magdalena Andersson told Swedish public television SVT, adding that a review of the country's security policy was underway.

"I want us to do a thorough analysis of our possibilities in this situation, of the threats and risks associated with them, to take the best decision for Sweden," the Social Democrat leader said.

Sweden is not a member of NATO and is officially militarily non-aligned, though it is a partner of the alliance. It abandoned its policy of neutrality after the end of the Cold War.

On February 24, the day Russia invaded Ukraine, Andersson had ruled out applying for membership.

"In a situation like this it is important that Sweden's long-standing security policy stays firm. That we are predictable and clear", she said at the time.

On March 8, she sparked debate when she said a Swedish NATO application risked "destabilising" the security situation in Northern Europe.

Moscow is vehemently opposed to Swedish or Finnish membership in the alliance.

In Sweden, support for NATO membership has soared since the start of the war in Ukraine, with around half of Swedes now in favour of joining, according to several opinion polls published since early March.

The share of those opposed to membership has fallen to around 25-30 percent.

"This is something that many Swedes are thinking about right now. I am obviously thinking a lot about it too," she said.

The issue is expected to be one of voters' top concerns in upcoming legislative elections in September.

Opposition leader Ulf Kristersson, of the right-wing Moderates party, has said he plans to submit a NATO membership application if he wins a majority in the election.

The far-right Sweden Democrats, currently the country's third-biggest party, have also said they are open to the idea of membership.

The prime minister's Social Democrats have historically been opposed to joining NATO.

However, if the current review were to conclude that it was in Sweden's best interest to abandon its policy of military non-alliance, Andersson said she would "of course" change her party's line.

The war in Ukraine has also led to a debate on NATO membership in neighbouring Finland, where a security policy review is also underway.