Putin Has 'Already Lost' Ukraine War, Biden Says

US President Joe Biden said Thursday that Russia's Vladimir Putin has already lost the war in Ukraine, expressing hope that Kyiv's counter-offensive would force Moscow to the negotiating table.

As Russia launched fresh strikes and a new bout of nuclear-sabre rattling, Biden said there was no real prospect of Putin using nuclear weapons and insisted the war would not drag on for years.

Biden also used a visit to Finland, NATO's newest member, to pledge that Ukraine would one day join the alliance, despite NATO leaders failing to give Kyiv a timeline at a key summit this week.

"Putin's already lost the war. Putin has a real problem," Biden told a press conference with Finnish President Sauli Niinisto. "There is no possibility of him winning the war in Ukraine."

NATO leaders had dashed Ukrainian leader Volodymyr Zelensky's hopes for a clear timeline to join the military alliance, saying at this week's summit in Vilnius that they would offer an invite only when "conditions are met".

But while Biden said no country could become a NATO member while it was at war -- with Ukraine joining now meaning a "third world war" -- he vowed Kyiv would one day join the club.

"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," Biden said.

Putin told journalists Thursday that if Ukraine were to be admitted to NATO, it would "in general make the world much more vulnerable" and boost global tensions.

Moscow also launched another aerial assault on Ukraine, injuring four people, while Kyiv said it had destroyed 20 Russian attack drones and two cruise missiles.

Senior Ukrainian military officials meanwhile said they had received controversial cluster munitions promised by the United States.

"We just got them, we haven't used them yet, but they can radically change (the battlefield)," Ukrainian army commander Oleksandr Tarnavskyi told US broadcaster CNN.

In a sign of its anger at Western backing for Kyiv, Moscow said it would regard F-16 fighter jets sent to Ukraine as a "nuclear" threat because of their capacity to carry atomic bombs.

"Russia cannot ignore the ability of these aircraft to carry nuclear weapons. No amount of assurances will help here," Foreign Minister Sergei Lavrov was quoted as saying by the Russian foreign ministry.

But Biden played down the latest bout of nuclear rhetoric from Russia.

"I don't think there's any real prospect... of Putin using nuclear weapons. Not only has the West but China and the rest of the world has said don't go there," he said.

Wagner chief Yevgeny Prigozhin should meanwhile be careful of poisoning following the mercenary group's failed uprising in Russia, Biden added.

"God only knows what he's likely to do. We're not even sure where he is and what relationship he has. If I were (him), I'd be careful what I ate. I'd keep my eye on my menu," Biden said in Helsinki.

Biden also said he was "serious" about the prospect of a prisoner exchange for jailed Wall Street Journal reporter Evan Gershkovich to get him home from Russia.

The US president was holding talks in the Finnish capital after G7 powers vowed to back Ukraine for as long as it takes to beat Russia.

Finland, which shares a 1,300 kilometre (800-mile) border with Russia, ended its historic military non-alignment and joined NATO following Moscow's invasion of Ukraine.

Biden and the leaders of Denmark, Finland, Iceland, Norway and Sweden pledged "unwavering" support for Ukraine in a joint statement after the talks.

They also discussed climate, which Biden called the "only existential threat humanity faces".

Biden is the first US president to visit Helsinki since Donald Trump's summit five years ago with Putin, and his press conference took place in the same hall.

He pledged that the United States would remain a member of NATO, after being asked about what would happen if Trump, who has raised the idea of pulling out of the alliance, is re-elected next year.

But Biden's stress on the strength and symbolism of NATO contrasted with events at the NATO summit, when Zelensky slammed the "absurd" decision not to fast-track Ukraine's membership.

Instead, G7 nations later offered Ukraine a package of long-term security commitments, involving bilateral deals between Kyiv and the world's richest nations.