Washington — Defense Secretary Lloyd Austin warned Sunday a chemical or biological weapons attack by Russia in Ukraine would prompt a "significant reaction" from the United States, as well as global allies and partners.

"If a chemical or biological weapon was used, you'd see a significant reaction from not only the United States, but also the global community," Austin said in an interview on "Face the Nation." "I don't want to speculate about what exactly would change our calculation. I think engaging in hypotheticals is probably not helpful here either, but this is a very serious step and as you heard our president say, we won't take that lightly."

Twenty-five days after Russia launched its invasion of Ukraine, Russian forces are struggling in their effort to take over the capital city of Kyiv but have continued their brutal bombardment of civilians.

In the port city of Mariupol, Ukrainian officials have accused Russia of bombing an art school where 400 people were sheltering, marking the second attack of a public building where civilians were taking shelter in less than a week. Russian forces struck a theater in Mariupol where more than 1,300 people were believed to be inside last week.

Outside of other major cities, Russia's advance has largely stalled, though there has been growing concern among Ukrainian and U.S. officials that Moscow could use chemical or biological weapons in the ongoing war.

The White House last week said Russia could escalate its attack by using such weapons or claiming Ukraine used chemical or biological weapons in a "false flag" attack as a pretext for more violence.

Russian officials, meanwhile, have accused the U.S. of working on secret biological and chemical programs inside Ukraine, a claim the White House has rebuked as absurd and part of a ploy by Russia to try to justify its attack on Ukraine.

Austin said that the U.S. has in the past seen Russia "raising issues and creating a pretext so that if they did something in the battle space, they could blame it on somebody else, either Ukrainians, us, NATO."

"This could possibly be what we're seeing today," he said.

Austin said the U.S. expects to see Russia's attacks on civilians in Ukraine continue as a result of its stalled campaign and in an attempt by Russian President Vladimir Putin to re-establish momentum.

"They're not being effective today in terms of their maneuver forces on the ground.

They're essentially stalled," he said. "It's had the effect of [Putin] moving his forces into a wood chipper."

Austin commended the Ukrainian people for their defense of the country against Russia, saying they have shown "significant resolve" and been effective at using the weapons and equipment provided by the U.S.

"What we've seen in the past is that the Ukrainians have counterattacked, that they've used their initiative to move things around on the battlefield and they've presented some significant problems for the Russians," he said.

The Biden administration has provided Ukraine with $2 billion in security assistance since 2021, and Congress earlier this month approved $13.6 billion in humanitarian and military aid to Ukraine and Eastern European allies.

Still, Ukrainian President Volodymyr Zelenskyy has urged the U.S. to help create a no-fly zone over his country's skies, a move President Biden has opposed due to concerns it would further escalate tensions with Russia.

Austin, though, said the U.S. will work with allies and partners to help create conditions for Ukraine to get equipment like Russian-made S-300 surface-to-air missile systems and "ensure they have the ability to protect their skies going forward."