Macedonia signed accession papers with NATO on Wednesday, a key step on the way to joining the alliance after resolving a long-running name row with Greece.
The Macedonian Foreign Minister Nikola Dimitrov and representatives of all current NATO members signed the protocol at alliance headquarters in Brussels.
NATO Secretary General Jens Stoltenberg said it was a "historic day" which will lead to Macedonia becoming the 30th alliance member once the deal has been ratified by all the other capitals.
"All #NATO Allies have signed the Accession Protocol ... which will bring more security & prosperity to the whole region. I look forward to the day when 30 flags will fly outside NATO HQ," Stoltenberg tweeted.
The agreement last month with Greece to change the name of the former Yugoslav republic to the Republic of North Macedonia ended one of the world's longest diplomatic disputes and paved the way for Skopje to join NATO and the European Union.
Macedonian Prime Minister Zoran Zaev said it was a "great, historic day for all our citizens".
"Today the largest security alliance in the world, NATO, has welcomed us," he wrote on Facebook and Twitter. "We have provided lasting stability for our country."
The EU's enlargement commissioner Johannes Hahn also tweeted congratulations, saying the move was "a contribution to peace and stability" in the Western Balkans.
Macedonia will now take part in NATO ministerial meetings as an invitee, Stoltenberg said, starting with a gathering of defence ministers in Brussels next week.
For Skopje to progress to full membership, all 29 current members must ratify the accession protocol. How long this will take depends on national procedures, but when Montenegro joined in 2017 it took about a year.
Dimitrov said Macedonia was determined to meet the NATO spending pledge of devoting at least two percent of GDP to defence by 2024.
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