"We encourage Turkey to also ratify Sweden's accession protocols quickly," added US President Joe Biden's top security adviser. Hungary must meanwhile complete its ratification process for the NATO accession of Finland and Sweden "without delay".

"Sweden and Finland are strong, capable partners who share NATO's values, will strengthen the alliance and contribute to European security," Sullivan said. The two Nordic countries should become NATO members "as soon as possible".

After months of delays, Erdogan gave the green light for Finland to join NATO on Friday. However, Turkey continues to block Sweden's membership and is demanding tougher action from the government in Stockholm against Kurdish activists in the country, whom Ankara describes as "terrorists".

After Russia's attack on Ukraine, Finland and Sweden gave up their decades-long policy of military neutrality and simultaneously applied for membership in the Western military alliance in May last year. All 30 NATO member states have to agree to this, only Turkey and Hungary have yet to say yes.