Turkish President Recep Tayyip Erdogan set out his conditions for backing Finland's and Sweden's NATO membership bids in a series of phone calls on Saturday.
He reiterated his insistence that his Western allies make concessions that address Ankara's security concerns and pave the way for new arms deals.
Sweden and Finland have both applied to join NATO, ending their neutral stance after Russia's ongoing invasion of Ukraine altered the security situation in Europe.
However, any new members must be unanimously approved by NATO's members.
Ankara will not take a positive approach to NATO's expansion unless Sweden and Finland support Turkey in "fundamental issues, particularly fighting terrorism," Erdogan told NATO Secretary General Jens Stoltenberg in a phone call on Saturday, his office said.
However, Erdogan also left room for a diplomatic solution, telling Stoltenberg that Ankara still supports NATO's open-door policy.
"We agree that the security concerns of all Allies must be taken into account and talks need to continue to find a solution," Stoltenberg tweeted after their call.
Earlier, Erdogan spoke separately with the leaders of Sweden and Finland, reiterating that he would only support their membership bids in exchange for an end to support for what Ankara sees as "terrorist" groups and an easing of restrictions on weapons purchases.
By terrorist groups, Erdogan was referring to the banned Kurdistan Workers' Party (PKK) and the Kurdish People's Defense Units (YPG) militia in Syria.
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The PKK, outlawed by the US and the EU, has been fighting for autonomy in Turkey for over 40 years. Turkish security forces also often clash with the YPG along the border and in Syria. Ankara believes the two groups are linked and fears they could spark a wider insurgency.
The US, by contrast, works closely with the YPG in Syria, considering it a key ally in its fight against Islamic State.
Sweden must cease its "political, financial and military support" for terrorist groups and end its "defense industry embargoes," Erdogan told Swedish Prime Minister Magdalena Andersson during their call, his office said.
Sweden and Finland, among others, imposed restrictions on arms exports to Turkey after a Turkish military offensive against the YPG in 2019.
Addressing these areas would show that Stockholm shares Ankara's security concerns, Erdogan said, according to his office.
Stockholm looks forward "to strengthening our bilateral relations, including on peace, security, and the fight against terrorism," Andersson's office tweeted later.
Erdogan also reiterated his claim that Sweden backs supporters of US-based Turkish cleric Fethullah Gülen, labelled a terrorist by Ankara, which holds him responsible for the attempted coup in 2016.
Ankara expects everyone to respect its "legitimate" fight against groups that "threaten Turkey's national security and its people," Erdogan said in his call with Finnish President Sauli Niinistö.
Despite Ankara's objections, other members of NATO, led by the US, strongly back the two countries joining the defense alliance and hope they will soon become members.
It was not immediately clear how Turkey could be persuaded to back Finland's and Sweden's accession plans or when a solution might be found.