As a result, NATO's governing body, the North Atlantic Council, was unable to begin the planned accession process straight away.
Sweden and Finland formally submitted applications for NATO membership to Secretary General Jens Stoltenberg in an online video ceremony on Wednesday.
President Recep Tayyip Erdogan said that Turkey was unable to accept a proposed enlargement of the alliance that would compromise NATO's own security, reiterating his claims that Sweden and Finland were effectively supporters of terrorism for their alleged support of the banned Kurdistan Workers' Party (PKK) and the Kurdish People's Defence Units (YPG) militia in Syria.
Turkey considers the PKK and YPG to be terrorist groups and sees them as existential threats to its security, often clashing with them along Turkey's border with Syria and Iraq.
The US, by contrast, works closely with the YPG in Syria, considering it a key ally in its fight against the Islamic State group.
In response to a Turkish military offensive against the YPG in 2019, Sweden, Finland and Germany, among others, imposed restrictions on arms exports to Turkey.
Finland and Sweden decided to apply for NATO membership following the Russian invasion of Ukraine and after a groundswell of public support in both countries accelerated the move.
NATO Secretary General Jens Stoltenberg said the alliance would assess the membership bid as quickly as possible but stressed that the security interests of all allies "have to be taken into account," referring to Turkey's objections.
Turkey could use its NATO card to push allies to label the YPG as a terrorist organisation while a clampdown on alleged PKK activities in Sweden is also on Turkey's agenda, Soner Cagaptay of the Washington Institute, a US foreign policy think tank, said.
Turkey stood little chance of winning any major concessions as the latest NATO expansion was "already sponsored by the US," former Turkish ambassador Uluç Özülker said.
Turkey had so far failed to produce concrete evidence of a PKK network in Sweden and Finland despite "a strong Kurdish diaspora with a strong political motivation in both countries," Salim Çevik of the SWP foreign policy institute in Berlin said.
Finnish and Swedish diplomats plan to travel to Ankara to discuss commitments on the fight against terrorism and potential arms deals, among others.
Turkish Foreign Minister Mevlüt Çavusoglu is scheduled to meet his US counterpart Antony Blinken in New York on Wednesday, according to Turkish state news agency Anadolu.
Should Turkey drop its objections, the accession of Sweden and Finland should be smooth.
Accession protocols could be signed in June, clearing the way for the ratification procedures, meaning both countries could be NATO members by the end of the year.
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