The accession of Sweden and Finland to NATO is "the fastest" in the alliance's history and will take months, rather than years, to complete, Secretary-General Jens Stoltenberg told Euronews.
The two Nordic countries signed their respective accession protocols on Tuesday, setting in motion a ratification process that has to go through the parliaments of all 30 member states.
"I'm very careful predicting or promising anything [about] parliaments. But last time, it took roughly a year and many allies have expressed that they can try to do it faster this time," Stoltenberg told Euronews.
"So hopefully we will most likely speak about months," he added. "There is a strong political will to ratify in the different allied countries."
A final unanimous vote is required to conclude the proceedings.
Denmark has already ratified the two protocols, while the Estonian Prime Minister has convened a parliamentary session on Wednesday to speed up the process.
Once inside the alliance, Sweden and Finland will benefit from the alliance's Article 5 of collective defence.
"President [Vladimir] Putin actually tried to close NATO's door," Stoltenberg said. "What we demonstrate today is that NATO's door remains open and that we stand together."
The momentous decision to enlarge the alliance was only made possible after last week Turkey agreed to lift its veto on Sweden and Finland's applications during a summit in Madrid.
Ankara gave up its opposition by signing a joint memorandum in which Stockholm and Helsinki commit to intensify the fight against the Kurdistan Workers' Party (PKK), remove arms embargoes and address Turkey's pending deportation requests "expeditiously and thoroughly."
The wording has raised concerns that Sweden and Finland offered too many concessions to Turkey, whose president, Recep Tayyip Erdoğan, called the deal a "diplomatic win" for this country.
"The document provides a good platform for Türkiye, Finland and Sweden to work even closer on fighting terrorism. That's a common goal that we all agreed on: the need to fight terrorism in all its forms and manifestations," said Stoltenberg, who moderated the talks between the three countries.
"No other NATO ally has suffered more from terrorism than Türkiye ."
'Brutal war of attrition'
In the interview with Euronews, the secretary-general also discussed the latest developments in Ukraine.
After days of intense fighting over the eastern city of Lysychansk, Russia has gained control of the Luhansk province, putting the country a step closer to controlling the entire Donbas region.
Moscow is now expected to focus on the areas of the neighbouring Donetsk province that are still under Ukrainian control. Kyiv continues to ask the West for the delivery of more modern heavy weapons that can help its forces resist a new Russian offensive.
"What we see now is a brutal war of attrition under President Putin. We see that, by using massive force, artillery, a brutal bombardment, they are killing people, they are destroying cities," Stoltenberg said.
"But at the same time, we also see that the strength and the bravery of the Ukrainian armed forces actually proves that Russia totally underestimated Ukraine," he noted.
"We have seen that Ukraine has also been able to launch some offensive operations, for instance, by liberating or retaking Snake Island."
Asked if Western sanctions were working to put the brakes on the Kremlin's war machine, Stoltenberg replied "absolutely."
"We see that the sanctions have a direct consequence for his economy. So, it is this combination of heavy sanctions imposed by NATO allies, the European Union, but also the significant [military] support," he said.
"President Putin also underestimated the unity and the will by NATO allies and partners to stand up."