The leaders of nine NATO nations from Central and Eastern Europe issued a joint statement on Sunday in support of Ukraine's path to membership in the alliance. 

"We firmly stand behind the 2008 Bucharest NATO Summit decision concerning Ukraine’s future membership," the presidents of Estonia, Latvia, Lithuania, Montenegro, Poland, Czechia, Romania, North Macedonia, and Slovakia said on Sunday. 

At that 2008 summit, NATO allies said they "welcomed" Ukraine and Georgia's aspirations to the join the alliance, though no clear timetable has ever been announced. 

Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, tweeted on Sunday that 10 NATO countries support Ukraine's bid for membership, including some nations that used to belong to the Soviet Union. 

"We are grateful for the leadership and responsibility," Podolyak tweeted. "History is being made today."

Ukrainian President Volodymyr Zelenskyy visits Mykolaiv region

In this photo provided by the Ukrainian Presidential Press Office, Ukrainian President Volodymyr Zelenskyy attends meeting with military officials as he visits the war-hit Mykolaiv region.  (Ukrainian Presidential Press Office via AP)

Zelenskyy said on Friday that Kyiv has "accelerated" its application for NATO membership after Russia annexed four Ukrainian territories. 

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"De facto, we have already proven compatibility with alliance standards. They are real for Ukraine – real on the battlefield and in all aspects of our interaction," Zelenskyy said. "We trust each other, we help each other, and we protect each other. This is the alliance."

NATO Secretary-General Jens Stoltenberg declined to comment on Ukraine's pathway to membership, telling NBC's "Meet the Press" on Sunday that any decision "has to be taken by consensus" but that the alliance's "top priority" is to support Ukraine. 

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White House national security adviser Jake Sullivan said Friday that the United States supports NATO's open-door policy for countries that want to join.  

"Right now, our view is that the best way for us to support Ukraine is through practical, on-the-ground support in Ukraine, and that the process in Brussels should be taken up at a different time," Sullivan said during a press conference.