A decision in favour of joining would in all likelihood pave the way for Sweden to submit a membership bid.

"The party will announce its position on May 15," which would be Sunday, party spokeswoman Julia Grabe told AFP.

If the Social Democrats support joining, there would be a clear parliamentary majority for an application, especially if neighbouring Finland -- where a decision is also expected in the coming days -- were to apply for membership of the US-led military alliance.

Sweden and Finland have been militarily non-aligned for decades, but public opinion in both countries has shifted following Russia's invasion of Ukraine, with support for membership soaring, according to polls.

Sweden's centre-left Social Democrats, led by Prime Minister Magdalena Andersson, has historically opposed NATO membership, and even reaffirmed this stance at the last party congress in November.

But the conflict in Ukraine has reignited debate in the country and within the party.

Finland's Social Democrats, which are also the ruling party, are expected to announce their position on May 14.

A Swedish parliamentary security policy review, conducted by all parties in parliament, is also due to be presented on May 13.

Both Sweden and Finland have close ties with the alliance, joining the Partnership for Peace Program in 1994 and regularly taking part in exercises with NATO countries and NATO-led peacekeeping missions.