Why does Turkey want to block Finland and Sweden from NATO?

The Turkish government's assertion that they have issues agreeing to let Finland and Sweden join NATO has raised eyebrows in Helsinki and Stockholm in a week where the Nordic nations are submitting historic letters requesting NATO membership. 

All 30 members of the military alliance must agree on admitting new members. 

But Turkish President Recep Tayyip Erdoğan described Finland and Sweden as a "hatchery" for terror groups. 

At the same time, Foreign Minister Mevlüt Çavuşoğlu said both countries must provide explicit security guarantees and lift export bans on some defence sector goods to Turkey.

"Our stance is perfectly open and clear. This is not a threat -- this is not a negotiation where we're trying to leverage our interests," Çavuşoğlu said.

The leader of a Turkish nationalist party also spoke out, saying that NATO’s expansion to include Sweden and Finland would provoke Russia and cause an expansion of the war in Ukraine. He urged Turkish lawmakers to keep both countries in "NATO's waiting room." 

Other NATO members have sought to downplay the Turkish threats -- and the Finns have been characteristically diplomatic about working out an amicable solution to any possible impasse. 

So what is behind the Turkish threats, and what is Erdoğan's possible end game here? 

Dr Paul Levin, the founding director of the Stockholm University Institute for Turkish Studies, told Euronews he thinks Turkey's primary concern is the presence of PKK activists in Sweden. 

"What Turkey perceives as threat of PKK is the core national security concern in Turkey. Sweden doesn't quite share the same perspective on that threat," said Levin. 

"So you know it is sort of natural opportunity, when Sweden now wants to join NATO, for Turkey to state its