Finland -- which shares a 1,300-kilometre (800-mile) border with Russia -- and Sweden applied to join NATO last year after Moscow's invasion of Ukraine, ending decades-long policies of military non-alignment.

"We have to assess the situation, whether something has happened that in the longer term would prevent Sweden from going ahead," Foreign Minister Pekka Haavisto told broadcaster Yle.

He added however that it was "too early to take a position on that now" and that a joint application remained the "first option".

Sweden's Foreign Minister Tobias Billstrom told media on Tuesday that he was "in contact with Finland to find out what this really means".

Haavisto later clarified his comments at a press conference, saying he did not want to "speculate" on Finland joining alone "as both countries seem to be making progress", and emphasising their commitment to a joint application.

But "of course, somewhere in the back of our minds, we are thinking about different worlds where some countries would be permanently barred from membership", he said.

The Danish-Swedish far-right politician Rasmus Paludan set fire to a copy of the Koran on Saturday in front of Turkey's embassy in the Swedish capital, angering Ankara and Muslim countries around the world.

"Sweden should not expect support from us for NATO," Turkey's President Recep Tayyip Erdogan said Monday.

"It is clear that those who caused such a disgrace in front of our country's embassy can no longer expect any benevolence from us regarding their application for NATO membership," Erdogan said.

Swedish leaders have roundly condemned the Koran burning but defended their country's broad definition of free speech.

The incident came just weeks after a support group for armed Kurdish groups in Syria, the Rojava Committee, hung an effigy of Erdogan by the ankles in front of Stockholm City Hall, sparking outrage in Ankara.

Haavisto said the anti-Turkey protests had "clearly put a brake on the progress" of the applications by Finland and Sweden to join the trans-Atlantic military alliance.

"My own assessment is that there will be a delay, which will certainly last until the Turkish elections in mid-May", Haavisto said.

- 'Plan B' out in the open -

Turkey has indicated in recent months that it has no major objections to Finland's entry into NATO.

Helsinki had refused until now to speculate on the option of joining without Sweden, emphasising the benefits of joint membership with its close neighbour.

But "frustration has grown in various corners of Helsinki", and "for the first time it was said out loud that there are other possibilities", Matti Pesu, a researcher at the Finnish Institute of International Affairs, told AFP.

"There has been a change" in the Finnish position, he said. "These Plan Bs are being said out loud."

Haavisto also accused the protesters of "playing with the security of Finland and Sweden", with actions that "are clearly intended to provoke Turkey".

"We are on a very dangerous path because the protests are clearly delaying Turkey's willingness and ability to get this matter through parliament," he said.

Pesu noted that while Turkey had so far given no indication it would treat the two applications "separately", it will be "interesting to see how Turkey reacts" to Haavisto's comments.

In his press conference, Haavisto denied that a "Plan B" existed.

"Such a path has not been seen as possible. It is very challenging to defend the North. Sweden has an important role to play in this," he noted.

Bids to join NATO must be ratified by all members of the alliance, of which Turkey is a member.

Ankara signed a memorandum of understanding with the two Nordic countries at the end of June, paving the way for the membership process to begin.

But Ankara says its demands remain unfulfilled, in particular for the extradition of Turkish citizens that Turkey wants to prosecute for "terrorism".