Finland and Sweden formally applied to join the NATO alliance on Wednesday at allied headquarters, a decision spurred by Russia's invasion of Ukraine, but face objections from Turkey to an accession process that is expected to take only a few weeks, Reuters reported.

Neutral throughout the Cold War, Sweden's and Finland's 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.

With the applications formally submitted, the Nordic countries and their many backers now face uncertain months where any resistance to their bids needs to be overcome, with all 30 of NATO's members needing to approve the enlargement.

Ratification by all 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, saying the two countries harbour individuals linked to groups it deems terrorists and hitting out at arms export embargoes imposed on it after its Syria incursion in 2019.

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