Finland and Sweden officially applied to join the NATO alliance Wednesday at allied headquarters, a move driven by security concerns over Russia's invasion of Ukraine.

The two countries' applications set in motion an accession process that is expected to take only a few weeks.

Sweden and Finland were both neutral throughout the Cold War and their decision to join NATO is one of the most significant changes in Europe's security architecture for decades, reflecting a sweeping shift in public opinion in the Nordic region since Russia's Feb. 24 invasion.

"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.

The application must now be weighed by the 30 member countries. That process is expected to take about two weeks, although Turkish President Recep Tayyip Erdoğan has expressed reservations about Finland and Sweden joining.

If his objections are overcome, and accession talks go as well as expected, the two could become members within a few months. The process usually takes eight to 12 months, but NATO wants to move quickly given the threat from Russia hanging over the Nordic countries' heads.

Canada, for example, says that it expects to ratify their accession protocol in just a few days.

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.