Finland must apply to join the NATO military alliance "without delay," Finnish President Sauli Niinisto and Prime Minister Sanna Marin said on Thursday, a major policy shift triggered by Russia's invasion of Ukraine.

Finland, which shares a 1,300 kilometer border and a difficult past with Russia, has gradually stepped up its cooperation with the bloc as a partner since Russia annexed Crimea in 2014.

But until Russia's invasion of Ukraine, the Nordic country had refrained from joining in order to maintain friendly relations with its eastern neighbor.