"When we see the problems coming, of course, we take this diplomatically. We are sending our delegations to visit Ankara from both Sweden and Finland. This will happen tomorrow," Haavisto told attendees at the World Economic Forum in Davos, Switzerland on Tuesday.

Sweden and Finland applied to join the transatlantic alliance in the wake of Russia's invasion of Ukraine.

And Spanish Prime Minister Pedro Sanchez said Tuesday that both countries would attend a June summit of the alliance.

"It's not just the war right now, this is a new Russia, this is a new environment," Swedish Finance Minister Mikael Damberg said.

"In the end, most people in the population, most political parties agreed that this was the time to see that we are more secure within NATO than outside."

Turkey's foreign ministry said presidential spokesman Ibrahim Kalin and deputy foreign minister Sedat Onal will meet the Finnish and Swedish officials on Wednesday.

Turkish Foreign Minister Mevlut Cavusoglu was quoted as saying, by private broadcaster NTV, that Ankara has prepared a "draft agreement" that will be the basis of the discussions.

Turkey wants "guarantees" that can be made in an official, signed agreement, not "wishes", he said.

NATO member Turkey has long accused Nordic countries, in particular Sweden which has a strong Turkish immigrant community, of harbouring outlawed Kurdish militants as well as supporters of Fethullah Gulen, the US-based preacher wanted over a failed 2016 coup in Turkey.

President Recep Tayyip Erdogan warned Saturday that Turkey would oppose membership for the two applicants unless its concerns were addressed -- potentially a major obstacle as a consensus is required in NATO decisions.

- 'NATO will be stronger' -

"We have very strict rules on any terrorist acts or any terrorist preparations in our country, and we take very seriously those limitations and rules also (set) by the European Union," Haavisto said.

"Now it's of course, our time to convince (Turkey) that our legislation is appropriate regarding the PKK (Kurdish separatists) and so forth," he added.

But Haavisto also warned that the Nordic countries' applications were a "question to NATO - is the NATO open door policy valid or not?"

"If there comes a moment when the open door policy is no longer valid then that's a big change," he said.

Beyond smoothing ruffles between the Nordic countries and Turkey, analysts say Ankara may be making a show of opposition to secure concessions from other NATO members, such as deliveries of fighter planes from the United States.

"I think it's very hard to speculate about the motives, because we got signals in the beginning of the process, before we went for our application, positive signals. So what changed in between? I don't know," Sweden's Damberg said.

"For us, taking these concerns seriously, we should do our homework and hopefully that could resolve the situation."

Nevertheless, he added that "I think NATO will be stronger as well with Sweden and Finland joining... that's also perhaps relevant for all the other members of NATO. Hopefully that plays a part."