Turkey has told Sweden to end its support for a Syrian-Kurdish militia or see its bid for NATO membership blocked by Ankara, according to reports this week.
Emre Yunt, Turkey's ambassador to Stockholm, told The Financial Times that Sweden must cut ties with the People's Protection Units (YPG) - which Ankara considers to be a "terrorist organisation" - or see its application for NATO membership vetoed.
Turkey said "the most important" demand on Sweden and Finland on their prospective NATO bid is the YPG.
"They have to cut their ties with YPG," Yunt told the financial daily. "That is the most important."
Alarmed by Russia's invasion of Ukraine, neutral Sweden and Finland have both officially requested to join NATO.
Turkey President Recept Tayyip Erdogen recently described Sweden as a "total terrorism centre, a nest of terrorism" after Stockholm and Helsinki turned down Ankara's request to extradite 30 individuals with alleged links to terror groups.
The YPG - which Turkey says is part of the banned Kurdistan Workers' Party (PKK) - led the fightback against the Islamic State group in Syria and is the dominant militia in the US-backed Syrian Democratic Forces (SDF).
The SDF's close relations with European countries and the US has been a key point of division between Turkey and its NATO allies.
Yunt says that this relationship has to be reviewed, particularly as IS "doesn't exist anymore".
The PKK has carried out bloody attacks on civilian and military targets in Turkey for decades.
Turkey’s foreign minister Mevlut Cavusoglu said that US Secretary of State Antony Blinken assured him he would pass Turkey's "legitimate security concerns" on to Sweden and Finland, the FT added.
"It's unacceptable that countries that want to be candidates support terrorist groups that target us," the foreign minister said.
"This isn’t just political support. For example, Sweden also provides weapons supposedly to fight against ISIS. They attack us with these weapons. Our security officers, our soldiers, our civilians are martyred with these weapons."
Sweden is hoping the US might push Turkey into allowing its ascension to NATO.