Turkey's president says he won't support Sweden and Finland joining NATO, claiming they are "home to many terrorist organisations".
Tayyip Erdogan has the power to block their entry, as Turkey's already part of the military alliance.
Sweden's expected to join Finland in the coming days in applying for membership, prompted by rising Russia aggression.
The Turkish premiere has thrown Sweden and Finland’s potential NATO membership into confusion, the two countries have stated that they wish to join the alliance on account of Russia's recent aggression in Ukraine.
Adding to this they share close proximity with the Kremlin and a land invasion would be possible if hostilities ensued.
Erdogan stated that last Friday that “We are following the developments regarding Sweden and Finland, but we don’t hold positive views,”.
For a new member state to join, they must first get consensus approval from all existing members.
Turkey joined NATO in 1952, and has the second-largest military in the 30-member alliance after the United States.
Erdogan also criticised NATO’s acceptance of Greece as a member in 1952 and called it a mistake.
Turkey and Greece are longtime rivals and have fought in conflicts against one another even as NATO members.