President Joe Biden defended his opposition to Ukraine's NATO membership amid its war with Russia, insisting no country can join the alliance while it is under attack.

"That guarantees that we're in a war — we're in a third world war," Biden told reporters Thursday in Helsinki, Finland. "It's not about whether or not they should or shouldn't join. It's about when they can join, and they will join NATO."

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At the same time, Biden was optimistic about Ukraine's prospects in the war, stating that Russian President Vladimir Putin "has already lost" and is operating under military and economic constraints. Biden also downplayed the possibility of Russia deploying nuclear weapons based on pressure from China and other partners.

"My hope is and my expectation is you'll see that Ukraine makes significant progress on their offensive, and that generates a negotiated settlement somewhere along the line," he said.

Biden added the United States was "not even sure" of Wagner leader Yevgeny Prigozhin's location after the mercenary led a failed uprising against Putin last month, but he was more certain about Russian attempts to meddle in U.S. politics.

"They're already interfering in American elections, they did that the last time," he said.

During a joint press conference with NATO Secretary General Jens Stoltenberg on the sidelines of the alliance's annual summit this week, Ukrainian President Volodymyr Zelensky expressed "gratitude" for NATO's continued support and its decision to relax certain conditions for Ukraine's membership. The day before, Zelensky had aired his frustrations with NATO over its decision not to extend Ukraine an invitation to become a member or specify a timeline or specific requirements.

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"President Zelensky, how soon after the war would you like to join NATO?" one U.S. reporter asked before Biden's bilateral sit-down with his Ukrainian counterpart.

"An hour and 20 minutes," Biden replied. "You guys ask really insightful questions."