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Biden says it 'remains to be seen' whether Ukraine will be admitted to NATO

Biden is scheduled to meet Wednesday with Russian President Vladimir Putin, who vehemently opposes the move.

President Joe Biden made clear Monday that Ukraine does not yet have the go-ahead to join the North Atlantic Treaty Organization, a move long sought by Ukraine and vehemently opposed by Russia.

For a moment, it appeared Ukraine may be closer to joining the trans-Atlantic mutual protection treaty when President Volodymyr Zelenskyy appeared to announce a change on Twitter shortly before Biden began a news conference in Brussels, where he was attending a NATO summit.

"Commend @NATO partners' understanding of all the risks and challenges we face. NATO leaders confirmed that (Ukraine) will become a member of the Alliance," Zelenskyy tweeted. His tweet did not say when the country would join.

Biden, who has previously called for Ukraine to join NATO, made clear that such a move had not been approved yet.

"School's out on that question. It remains to be seen," he said when asked for a "yes or no" about Ukraine joining the organization. "In the meantime we will do all we can to put Ukraine in a position to be able to continue to resist Russian physical aggression."

"It depends on whether they meet the criteria. The fact is they still have to clean up corruption and the fact is they have to meet other criteria to get into the action plan."

Biden is set to meet Wednesday with Russian President Vladimir Putin, who's voiced opposition to Ukraine joining NATO in the past.

NATO nations have a mutual-defense pact, meaning if one nation is attacked, the others will join in the response. It's a provision that has been invoked only once, after the Sept. 11 terrorist attacks on the U.S.

In his opening remarks, Biden said NATO will stand behind Ukraine's "sovereignty and territorial integrity," but made no mention of the country joining the organization.

Biden said he'll convey to Putin "that I'm not looking for conflict with Russia, but that we will respond if Russia continues its harmful activities. And we will not fail to defend the trans-Atlantic alliance or stand up for democratic values."

Biden has a long history with Ukraine, and was tasked with pushing the country to fight corruption during the Obama administration.

During the same time frame, Biden’s son Hunter joined the board of a Ukrainian energy company, Burisma, which led then-President Donald Trump to urge Zelenskyy to open up an investigation into the Bidens for corruption in the run-up to the 2020 election.

Trump was impeached by the House over the phone call, but was acquitted in the Senate. Trump described the call as "perfect."

Russia and Ukraine have been locked in conflict since 2014, when Russia annexed Ukraine's Crimea Peninsula and began supporting separatists in the country's east.

In March, Russia started sending thousands of troops, tanks, artillery and other units to Crimea and regions along its 1,200-mile land border with Ukraine, Western governments and independent experts who monitor these maneuvers have told NBC News.

There are 30 countries in NATO, which was formed in 1949 to counter the Soviet Union.

The alliance is best known for Article 5 of its founding document: "An armed attack against one shall be considered an attack against them all."