NATO Secretary-General Jens Stoltenberg said on Wednesday that he "cannot guarantee" that Finland and Sweden will join the alliance quickly.

During the "NATO in a changed world" forum, Stoltenberg stated that “this is not the first time we see one or just a few allies not agreeing with the rest, but they applied for not so many weeks ago. My aim is to make sure that they can join soon, I can not guarantee, but this is still my aim."

According to Stoltenberg, in addition to agreeing on the admission of the two Nordic states, NATO will need to hold in-depth negotiations and resolve the concerns of the alliance's diverse member countries.

“I am confident that we also agree on Finland and Sweden but then we have to do what we always do in NATO and that is to sit down and address differences and concerns when they are clearly expressed by allies as Turkey has expressed now,” Stoltenberg added.

On May 18, three months after the Ukraine crisis began, Finland and Sweden submitted NATO membership bids, ending decades of neutrality. However, Turkey refused their applications, and President Tayyip Erdogan stated that Ankara could not consent as long as they supported Kurdish "terrorists".

Read next: Turkey: NATO summit not deadline for talks with Sweden, Finland

To call Russia "top threat" formally

NATO Secretary-General Jens Stoltenberg also stated that Russia would be formally called the alliance's "top threat" in the alliance's new Strategic Concept, which is scheduled to be endorsed at the next Madrid summit.

The NATO Strategic Concept is the organization's key document, in which member countries reaffirm NATO's values and purpose, provide a collective assessment of the security environment, define security challenges and measures to address them, and outline guidelines for the alliance's future political and military development.

"We will agree on a new strategic concept for NATO to guide NATO in a radically changed security environment, I expect the 2022 concept will refer to Russia as the most significant and direct threat to our security," Stoltenberg announced.

Stoltenberg had announced days ago that NATO's new strategy would not consider Russia as a strategic partner but rather as a threat "to peace and stability," In addition, the new NATO strategic concept will also mention China, whose rise poses a threat to NATO's interests, values, and security, he added.

On his part, Kremlin Spokesperson Dmitry Peskov said in early April that further expansion of the alliance eastward is aggressive in nature and will not make Europe more secure.

Madrid will host the NATO summit on June 28-30.