“This is a historic moment, which we must seize,” NATO Secretary-General Jens Stoltenberg said at a short ceremony in which the Swedish and Finnish ambassadors to the alliance handed over their application letters, each in a white folder embossed with their national flag.

“I warmly welcome requests by Finland and Sweden to join NATO. You are our closest partners, and your membership in NATO will increase our shared security,” Stoltenberg said. The alliance considers that the accession of Finland and Sweden would hugely strengthen it in the Baltic Sea.

Ratification of all 30 allied parliaments could take up to a year, diplomats say.

Turkey has surprised its allies in recent days by saying it had reservations about Finnish and Swedish membership. Stoltenberg said on Wednesday that he thought the issues could be resolved.

“We are determined to work through all issues and reach rapid conclusions,” Stoltenberg said, noting strong support from all other allies.