(NewsNation) — Although it is “unlikely” that Russian President Vladimir Putin will actually use nuclear weapons in Ukraine as he threatened to, U.S. officials still need to take what he said seriously, John Hardie of the Foundation for Defense of Democracies said.

As Moscow is poised to annex swaths of Ukraine, Putin declared he won’t hesitate to use nuclear weapons to protect Russian territory. He also raised the stakes by announcing a partial mobilization, vowing to use “all available means” to deter future attacks against Russia.

“The aim presumably would try to try to end the war and get Ukraine to accept Russia’s absorption of the occupied territories,” Hardie said on NewsNation’s “Morning in America.”

Hardie, deputy director of the foundation’s Russia program, said it is unclear whether Putin’s serious about these nuclear threats, and also how the U.S. would respond.

But even though there’s a low probability of actual nuclear use, the potential consequences are “so enormous they can’t be dismissed,” Hardie said.

“It’s really an open question,” he added. “But I wish the White House and company luck in solving it because it is a tough policy challenge.”

On Wednesday, President Joe Biden had harsh words for Russia, saying the country violated the United Nations charter with its “brutal, needless war” in Ukraine, and that it is an affront to what the U.N. stands for.

Addressing the U.N. General Assembly, Biden said Putin’s nuclear threats against Europe show “reckless disregard” for Russia’s responsibilities as a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons.

He called on all nations to speak out against the Russia’s invasion and help Ukraine defend itself.

“We will stand in solidarity against Russia’s aggression, period,” Biden said.

Hardy said Biden’s speech was strong.

“I liked the tone,” Hardie said. “We need to consistently remind the national community that Russia started this war — that its aims are, in some ways, even genocidal.”

The Associated Press contributed to this story.