His statement leads the Anadolu agency.
According to Stoltenberg, Sweden and Finland, as invited countries, will be able to take part in the alliance's summit in Madrid, which will be held on June 28-30.
At the same time, he noted that it would be "difficult" for them to consolidate their aspirations as candidate countries if they do not respond to Turkey's demands by June 28.
Earlier, Turkish Foreign Minister Mevlut Cavusoglu said that Turkey wants to receive written guarantees from Finland and Sweden regarding the resolution of conflicts before these countries join NATO.