Nato Secretary General Jens Stoltenberg said that Finland and Sweden are in a stronger position now than they were before they applied to join the military alliance last year.
The Nato chief said it would be impossible for alliance members not to intervene if Finland's or Sweden's security were to be threatened.
He made the comments on Tuesday at a joint press briefing in Helsinki with Finnish Prime Minister Sanna Marin, alongside a Nordic Social Democratic parties meeting.
Stoltenberg reiterated that it is up to each Nato member state to decide about ratifying the Nordic countries' alliance membership, but that there were indications that Hungary will soon do so.
He said that the application ratification process is moving forward, noting that it was a good sign that Turkey's discussions with Sweden and Finland are scheduled to resume next week.
Marin: Delays test alliance's credibility
Turkey and Hungary are the only Nato countries yet to ratify the Nordic countries' applications to join the alliance.
The alliance chief said that the two Nordic countries have fulfilled the criteria for membership and that it was time that their accession was ratified. But Stoltenberg said he was unable to offer a timetable on when that might happen.
Meanwhile, PM Marin said that the delays in ratifying Finland and Sweden's Nato applications are testing the alliance's open door policy and threatening its credibility.
The PM also said she thinks that Finland and Sweden will be accepted as alliance members before Nato's summit in Vilnius, Lithuania in July.
Marin and Stoltenberg spent part of the day with Nordic leaders for the annual meeting of SAMAK, the Cooperation Committee of the Nordic Social Democratic parties and trade union labour organisations.
Stoltenberg previously served as Prime Minister of Norway as a member of the social democrats.