Turkish President Recep Tayyip Erdoğan on Monday set a new condition for approving Sweden's NATO membership, calling on European countries to "open the way" for Turkey to join the European Union.

Erdoğan's sudden announcement before leaving to attend the NATO summit in the Lithuanian capital added a new layer of uncertainty to Sweden's attempt to become the thirty-second member of the alliance, which Turkey initially blocked, saying Sweden was too lenient with Kurdish militants and other groups Ankara considers security threats.

This was the first time Erdoğan linked his country's ambition to join the European Union with Sweden's efforts to become a NATO member. Speaking to reporters in Istanbul, Erdoğan said, "Turkey has been waiting at the door of the European Union for more than 50 years, and nearly all NATO member countries are now members of the European Union. I am directing this call to these countries that have kept Turkey waiting at the doors of the European Union for more than 50 years."

Erdoğan continued, "Come and open the way for Turkey's membership in the European Union. When the way is paved for Turkey, we will pave the way for Sweden as we did with Finland."

Earlier, Erdoğan's office said he informed U.S. President Joe Biden during a phone call on Sunday that Turkey wants a clear and strong message supporting Turkey's ambitions in the European Union from the NATO leaders' meeting in Vilnius. The White House reading of the Biden-Erdoğan call did not mention Turkey's European Union membership issue.

Erdoğan and Swedish Prime Minister Ulf Kristersson are expected to meet later on Monday in Vilnius.

In response to a question about Erdoğan's comments, NATO Secretary General Jens Stoltenberg said he supports Turkey's ambition to join the European Union, but noted that it was not among the conditions included in an agreement signed by Sweden, Finland, and Turkey at last year's NATO summit in Madrid.

Stoltenberg confirmed that Sweden has met those conditions and said he believes a positive decision on the country's pending membership could still be made during this week's summit in Lithuania.

European Commission spokeswoman Dana Spinant said, "You cannot link the two processes regarding Turkey."

Turkey is a candidate to join the European Union, but democratic backsliding during Erdoğan's presidency, disputes with Cyprus, an EU member, and other issues have hampered the country's progress towards joining the 27-nation bloc.

However, as a NATO member, Erdoğan's government has delayed ratifying Sweden's accession to the alliance, saying the administration in Stockholm needs to do more to suppress Kurdish militants and other groups. A series of anti-Turkey and anti-Islam protests in the Swedish capital raised doubts about the possibility of reaching an agreement to meet Turkey's demands before the alliance summit.

Turkey's delay of Sweden's accession sparked resentment among other NATO allies, including the United States, where Biden's national security advisor Jake Sullivan on Sunday confirmed that Biden and Erdoğan spoke earlier that day about Sweden's NATO membership among other issues and agreed to meet in Vilnius for further talks. Sullivan did not mention the EU membership issue.

Sullivan said, "The White House is confident Sweden will join the alliance, we are confident that will happen, we don’t consider that something basically in doubt, it’s a matter of time."

Erdoğan's recent statements shocked veteran Turkey analysts. Paul Levin, director of the Turkish Studies Institute at Stockholm University, said, "Erdoğan has made new demands and shifted the goalposts repeatedly during this process, but trying to pressure the European Union on the NATO matter is somewhat astonishing."

However, he added, "I think we should interpret his remarks cautiously for now, they can indicate everything from paving the way for face-saving consent for Sweden to trying to sabotage NATO's expansion by raising impossible demands. What can be said is that if he indeed conditions Sweden's NATO accession on restarting Turkey's EU accession process, it is unlikely that Sweden will become a NATO ally anytime soon."

Before Erdoğan’s comments, Swedish Foreign Minister Tobias Billström expressed optimism that Turkey would drop its objections to Stockholm’s NATO bid, saying, "What we rely on, of course, is reaching a point where we get a message from President Erdoğan that there will be what you can call a green light, a message indicating that the ratification process in Turkey can start in parliament."

Tobias Billström emphasized that Sweden has fulfilled its part of the agreement with Finland and Turkey, which included lifting the arms embargo on Turkey, tightening anti-terrorism laws, and intensifying efforts to prevent activities of the banned Kurdistan Workers' Party or PKK, which has waged a rebellion in Turkey since 1980.