Turkish President Recep Tayyip Erdogan once again declared his opposition to Finland and Sweden joining NATO and signaled Turkey might block their admission.

Finnish and Swedish leaders officially applied for NATO membership on Wednesday, May 18, ending decades of formal neutrality, assuming they are admitted. The two Nordic nations increasingly see their security as dependent on NATO following Russia’s invasion of Ukraine in late February.

Erdogan objected to their admission, accusing both countries of harboring Kurdish separatist groups his government considers “terrorist organizations.”

He stated, “Neither country has an open, clear stance against terrorist organizations.”

The source of tension lies in Turkey’s decades-long conflict with its Kurdish population and Kurds in the region trying to establish their own nation-state.

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During the Syrian Civil War, western governments supported Kurdish fighters against ISIS. Ankara believes these fighters have ties to the Kurdistan Workers’ Party, an armed separatist group operating in southeastern Turkey.

Ankara has in the past demanded that suspected Kurdish militants living within Finland and Sweden be extradited to Turkey. Both countries refused.

Tensions between Turkey and the two Nordic nations also flared up in 2019 when Finland and Sweden imposed sanctions on Turkey, suspending weapon sales to the country over its incursion into northern Syria to attack Kurdish fighters.

“We cannot say ‘yes’ to those who impose sanctions Turkey,” said Erdogan.

Turkey’s Foreign Minister Mevlut Cavusoglu told reporters in Berlin, “There absolutely needs to be security guarantees here. [Finland and Sweden] need to stop supporting terrorist organizations.”

Because admitting new countries to NATO requires unanimous consent from current member states, Erdogan’s decision is widely seen as a Turkish attempt to receive concessions before agreeing to vote in favor, according to Business Insider.

Reuters reported that Finnish President Sauli Niinisto spoke with Erdogan on the phone a month ago and that he had initially been supportive of the idea of Finland and Sweden joining NATO.

“Statements from Turkey have very quickly changed and become harder during the last few days,” said Niinisto, addressing Sweden’s parliament. “But I am sure that, with the help of constructive discussions, we will solve the situation.”