Sweden will formally apply for NATO membership in the next few days, Prime Minister Magdalena Andersson has said, but its accession process, and that of Finland, hit a snag when NATO member Turkey's president said he would not approve either bid.
Sweden and Finland need each of NATO's 30 members to approve their applications.
The ratification process had been expected to take up to a year, though Turkey's objections have thrown that into doubt.
At a news conference, Turkish President Tayyip Erdogan said Sweden and Finland should not bother sending delegations to Ankara to persuade Turkey to support for their bids.
"Neither of these countries have a clear, open attitude towards terrorist organisations," Mr Erdogan said.
"How can we trust them?"
He called Sweden a "hatchery" for terrorist organisations with terrorists in parliament.
Russia's invasion of Ukraine has shaken up Europe's security architecture and forced Sweden and Finland to choose sides after staying out of the US-led NATO alliance during the Cold War.
Sweden's ruling Social Democrats dropped their 73-year opposition to joining NATO yesterday and are hoping for a quick accession, abandoning decades of military non-alignment following Russia's invasion of Ukraine in February.
But Turkey's objections, which NATO leaders initially hoped would not cause a major delay, now look to present a serious obstacle.
A spokesperson for Swedish Foreign Minister Ann Linde declined to comment.
The decisions by Finland and Sweden to apply to NATO set the two countries on a path toward ending policies of military non-alignment that had defined their defence strategies since the start of the Cold War.
"We are leaving one era behind us and entering a new one," Ms Andersson told a news conference on Monday.
She said Sweden's application could be submitted in the next couple of days and would be synchronised with Finland.
"NATO will strengthen Sweden, Sweden will strengthen NATO," she said.
The decision to abandon the military non-alignment that has been a central tenet of Swedish national identity for two centuries reflects a sea change in public perception in the Nordic region following Russia's attack on Ukraine.
Ms Andersson said Sweden did not want permanent NATO military bases or nuclear weapons on its territory if its membership was approved.
Russian President Vladimir Putin offered a mild response to the decisions, saying: "As far as expansion goes, including new members Finland and Sweden, Russia has no problems with these states - none."
He did however accuse the United States of using the enlargement in an "aggressive" way to aggravate an already difficult global security situation.
He said Russia would respond if the alliance moves weapons or troops forward. General Micael Byden, commander of the Swedish Armed Forces, told a news conference the decision to apply was right from a military strategic perspective and that defending Sweden, unilaterally or in cooperation with other states, would be easier with Sweden part of NATO.
"I know, based on my conversations and the relations that I have with my counterparts, that Sweden is welcome in NATO. But we are not only welcome - I also know that Sweden as a member makes NATO stronger," Mr Byden said.
Sweden has received assurances of support from the United States, the United Kingdom, Germany and France but not any legally binding guarantees of military aid.
In a joint statement earlier, Nordic neighbours Denmark, Norway and Iceland also pledged support.
Moscow calls its invasion of Ukraine a "special military operation" to rid the country of fascists, an assertion Kyiv and its allies say is a baseless pretext for an unprovoked war.
Earlier, Russian President Vladimir Putin said Russia had no issue with Finland and Sweden, but that the expansion of military infrastructure on their territory would demand a reaction from Moscow.
Speaking in Moscow at a summit of the Russian-led Collective Security Treaty Organisation (CSTO), Mr Putin said that NATO's expansion was a problem for Russia and that it must look closely at what he said were the US-led military alliance's plans to increase its global influence.
Russian Deputy Foreign Minister Sergei Ryabkov meanwhile warned that the bids to join the NATO military alliance were serious mistakes.
Mr Ryabkov said Finland and Sweden should have no illusions that Russia will simply put up with their joining NATO.
"This is another grave mistake with far-reaching consequences," Mr Ryabkov was quoted as saying by Russian news agencies.
"The fact that the security of Sweden and Finland will not be strengthened as a result of this decision is very clear to us.
"The general level of military tension will rise, predictability in this sphere will decrease. It is a shame that common sense is being sacrificed to some phantom provision about what should be done in this unfolding situation," he added.
Russia has warned Finland, with which it shares a 1,300 kilometre border, that it would take "reciprocal steps".
Finnish President Sauli Niinisto spoke with Mr Putin on Saturday about the country's application for NATO membership.
The Kremlin said Mr Putin viewed any end to Finland's military neutrality as a "mistake".