“My congratulations to Finland, Sweden and Turkey on signing the tripartite memorandum is an important step towards inviting Finland and Sweden to NATO,” Biden tweeted.

He noted that the expansion of NATO will strengthen the collective security of the alliance.

Earlier, Finnish President Sauli Niinistö's office said Turkey would support Finland and Sweden's NATO membership at the Madrid summit this week.

NATO Secretary General Jens Stoltenberg said that the leaders of the member countries of the alliance will decide on June 29 to invite Sweden and Finland to become members of the military association.

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