NATO, together with Sweden and Finland is looking for ways to address Turkey's concerns about the membership of these countries in the alliance, said Thursday the organization's Secretary-General Jens Stoltenberg.

The secretary-general said that NATO takes seriously Turkey's concerns about Sweden and Finland's membership, which is at the same time a "historic" step.

Finland and Sweden submitted applications to the NATO secretary-general for membership in the alliance against the backdrop of the events in Ukraine on 18 May.

Turkey has blocked the start of the application process. Turkish President Tayyip Erdogan said Ankara could not say yes to Finland and Sweden's membership in NATO because it could not believe their assurances over links to representatives of the Kurdistan Workers' Party, which is banned in Turkey.