Stubb said the "logic of deterrence" and "strategic stability between the superpowers" were undergoing a transformation.

"We have entered a new era of nuclear weapons, in which, unfortunately, the significance of nuclear weapons has been increasing," Stubb said in a speech in Helsinki.

Last week, US President Donald Trump announced that the United States will begin testing nuclear weapons, raising questions about whether he meant conducting the country's first nuclear explosion since 1992.

The announcement came in the wake of Russia saying it had tested a new nuclear-powered cruise missile, the Burevestnik, and a nuclear-powered and nuclear-capable underwater drone.

No country other than North Korea is known to have conducted a nuclear detonation for decades.

Russia and China have not carried out such tests since 1990 and 1996 respectively.

On Sunday, Trump alleged that countries including Russia and China have conducted underground nuclear tests unknown to the public, and that the United States would follow suit.

"How can we build deterrence together? How can we control escalation?" Stubb said were questions a small country like Finland, which shares a 1,340-kilometre (830-mile) border with Russia, now needed to consider together with allies.

Dropping decades of military non-alignment, the Nordic country joined the US-led NATO military alliance in 2023, following Russia's invasion of Ukraine.