Finland and Sweden have both now confirmed their bid to join the North Atlantic Treaty Organization (NATO) after many decades of military neutrality.

However, Turkey has surprised its NATO allies by saying it would not view the Nordic neighbors' accession positively after President Tayyip Erdogan said the countries were "guesthouses for terrorist organizations."

With any decision on NATO enlargement needing approval from all allies, Turkey's position endangers what could be a major boon to the U.S.-led military bloc.

Here's what you need to know about Ankara's ultimatum – and whether it could put an end to Finland and Sweden's NATO bids.

READ MORE

What is NATO and why is Russia worried?

Boxing away stress in Kyiv

What can Finland's military offer NATO?

Why is Turkey stalling the talks?

Up until Friday, the two Nordic nations' NATO bids were expected to be roundly accepted by all 30 members, with the bloc pushing through their applications at an unprecedented speed. 

However, on Friday the Turkish president unexpectedly hit out at Finland and Sweden, claiming they served as "safe havens for terrorists of the PKK."

The Kurdistan Workers' Party (PKK), which has led a decades-long insurgency in Turkey, is blacklisted as a terrorist organization by Ankara, the U.S. and the EU. 

Turkey has repeatedly slammed Sweden and other Western countries for their handling of Kurdish separatist organizations, including the Kurdish-led Syrian Democratic Forces (SFD), which is fighting Turkish-backed Islamist forces in Syria.

Ankara says the Western-backed SDF is an extension of the PKK and has accused Sweden of allowing government-sanctioned fund-raising and recruiting for the Kurdish militia and its allies.

Speaking during NATO talks in Berlin, foreign minister Mevlut Cavusoglu said such support was "unacceptable and outrageous".

"The fact that these two countries are in contact with members of a terrorist organization, that Sweden sends them weapons and that they impose restrictions on the export of defence equipment to Turkey, goes against the spirit of alliance," he said. 

Another concern for Turkey is the Nordic nations' alleged protection of followers of the U.S.-based Islamic cleric Fethullah Gulen. Ankara says Gulenists carried out a coup attempt in 2016, which Gulen and his supporters deny.

READ MORE: What must Finland and Sweden do to become NATO members?

What does Turkey want?

Laying out its demands on Sunday on the sidelines of the Berlin meeting, Ankara wants the Nordic countries to halt support for Kurdish groups present on their territory, and lift bans on the sales of some weapons to Turkey.

It is also apparently calling for the repatriation of scores of Nordic residents with alleged links to Gulen and the PKK. Cavusoglu said he was waiting to see what guarantees would be offered by the two countries. 

"It is absolutely necessary to end support for terrorist organizations and lift restrictions on exports to Turkey. I'm not saying that as a bargaining chip, but because that's what it means to be allies," Cavusoglu said. 

He added that Turkey was not threatening anybody or seeking leverage, but speaking out about potential allies' support for an enemy, a direct contradiction of Article 5 of NATO's founding treaty, which says an attack on any NATO country should be seen as an attack on all.

Are Sweden and Finland prepared to satisfy those demands?

So far, Cavusoglu has applauded Finland's "very respectful" approach, but hit out at Sweden for "provocative" statements during the Berlin talks.

"The Swedish foreign minister's statements are unfortunately not constructive. She continues to make provocative remarks," Cavusoglu said.

Sweden's top diplomat Ann Linde has stressed that Stockholm considers the PKK a terrorist organization but there remained "misunderstandings" over using the term for other Kurdish groups, such as the YPG.

Sweden and Finland have also said they will not approve the repatriation of 33 people that Turkey has accused of links to terrorism. Nineteen requests were rejected, five left without a response, and nine are still being evaluated.

"We will send a group of diplomats to hold discussions and have a dialogue with Turkey," Swedish Defense Minister Peter Hultqvist told public service broadcaster SVT over the weekend, "So we can see how this can be resolved and what this is really about."

READ MORE: U.S. weaponizing sanctions waivers against Syria

How strong is Turkey's opposition and is it really about the PKK?

Turkey says it has "not shut the door" to Sweden and Finland joining NATO, with Erdogan's spokesperson Ibrahim Kalin saying that membership was "always a process" and they would "see how things go."

Asked whether Turkey risked being too transactional at a time of war, and when Finnish and Swedish public opinion favors NATO membership, he said: "If they have a public concerned about their own national security, we have a public that is equally concerned about our own security. We have to see this from a mutual point of view."

However, Ankara has been accused of using the issue not just to extract gains from the prospective NATO members, but also from the U.S., the de facto leader of the bloc.

Ankara and Washington's complicated relationship, in part due to U.S. support for Kurdish fighters in the Syrian War, has been further strained by Turkey's controversial purchase of Russia's S-400 weapons systems and its decision to maintain regular relations with Moscow amid the Ukraine crisis.

With the Biden administration keeping Erdogan at arm's length, talks over Finland and Sweden's accession are less likely to hinge on the Nordic nations' treatment of Kurdish separatists, and more on Turkey being welcomed back into the Western fold. 

READ MORE: What's behind Turkey's latest Kurdish offensive

Could Turkey stop them joining?

Turkey has the ability to veto the countries' memberships, but Ankara will be under immense pressure to accept the bids, NATO diplomats said, because the alliance considers that the accession of Finland and Sweden would hugely strengthen it in the Baltic Sea.

"I'm confident that we will be able to address the concerns that Turkey has expressed in a way that doesn't delay the membership," NATO Secretary-General Jens Stoltenberg said.

One of the bargaining tools the U.S. has is the potential sale of 40 F-16 fighter jets to Turkey, which U.S. Congress had previously opposed.

Erdogan's spokesperson said this weekend that "the Biden administration has taken a number of concrete, positive steps to move this thing forward" and receive congressional approval for the deal, which could help push through Finland and Sweden's accession.

U.S. Secretary of State Antony Blinken declined to go into details of closed-door conversations in Berlin but echoed Stoltenberg's position.

"I'm very confident that we will reach consensus on that," Blinken told reporters, adding that NATO was "a place for dialogue".