A key meeting between President Recep Tayyip Erdoğan, NATO chief Jens Stoltenberg, Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinistö has started in Madrid in a bid to advance talks on the Nordic countries' membership bids.

"We do not want empty words, we want results," Erdoğan told reporters in Ankara ahead of his departure for a NATO leaders summit in Madrid, adding that he was tired of waiting.

"If Sweden and Finland are to become NATO members, they have to take the security concerns of Turkey seriously, which has been a member of the alliance for 70 years," Erdoğan said.

Russia’s war on Ukraine prompted Finland and Sweden to formally apply to join NATO on May 18.

But Turkey, a longstanding member of the alliance, has voiced objections to their membership bids, criticizing the countries for tolerating and even supporting terrorist groups such as the PKK and its Syrian wing, the YPG, as well as for weapons embargoes against the country.

While the two Nordic countries said talks to resolve the dispute would continue, Erdoğan said recently that Ankara had not received any responses to its demands, including stopping support for terrorist groups, lifting the arms embargo on Ankara and extraditing terrorism suspects it seeks.

Any bid to join NATO requires unanimous backing from each of its 30 members.

Ankara's main demands are for the Nordic countries to halt support for the PKK and the YPG groups present on their territory, and to lift their bans on some sales of arms to Turkey. Ankara says the arms ban is inappropriate for prospective members of a security pact.

Listed as a terrorist group by Turkey, the United States and the European Union – of which Sweden and Finland are members – the PKK has waged an insurgency against Turkey since 1984.