Russian President Vladimir Putin’s campaign in Ukraine is "stalled," U.S. Secretary of Defense Lloyd Austin said Sunday, categorizing Moscow’s strategy as feeding their own troops into "a wood chipper."

"Their maneuver forces on the ground are essentially stalled," Austin said Sunday during an appearance on CBS’ "Face The Nation." "It's had the effect of him moving his forces into a wood chipper. You know, the Ukrainians have continued to attrit his forces, and they've been very effective, using the equipment that we provided them, you know, and armor weapons and aircraft weapons." 

Austin, who recently returned from a trip to Bulgaria and Slovakia, said Putin has not been able to achieve his goals as rapidly as he anticipated, so he’s resorted to targeting cities, towns and civilians more heavily. 

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Western officials and analysts said the conflict in Ukraine has resorted to a war of attrition, with Russia bombarding cities. Ukrainian officials defiantly rejected a Russian demand that their forces in Mariupol lay down their arms and raise white flags Monday in exchange for safe passage out of the besieged port. The rejection came after Ukrainian officials said an art school where some 400 people were taking shelter hit by an airstrike, prompting Ukrainian President Volodymyr Zelenskyy to vow to "shoot down the pilot who dropped that bomb."

U.S. Secretary of Defense Lloyd Austin reacts during joint news conference with Bulgarian Prime Minister in the Council of Ministers, Sofia, Saturday, March 19, 2022. This is the first visit of the US Secretary of Defense to Bulgaria in the last 25 years.  ((AP Photo/Valentina Petrova))

Reacting to Russian claims that their troops used hypersonic missiles at least twice in Ukraine so far, Austin said he did not view the move as a "game changer," because from his perspective, Russian forces having suffered heavy casualties might be using these types of weapons to desperately reestablish momentum. 

Austin also confirmed that the U.S. has heard from "a number of sources" that Russia is sending in "reinforcements" of Kremlin-backed mercenaries into Ukraine, stressing they have not seen Russian mercenaries on battlefields so far. Though he did not name a specific group of mercenaries, his remarks came after the Main Intelligence Directorate of the Ministry of Defense of Ukraine warned another group of Wagner Group began arriving in Ukraine on Sunday with orders from Putin to eliminate Zelenskyy, Ukrainian Prime Minister Denys Shmyhal and Andriy Ermak, Zelenskyy’s chief adviser. 

Defense Secretary Lloyd Austin speaks with U.S. troops, Friday, March 18, 2022 at an Army training range in Bulgaria.  Austin was in Bulgaria to meet with U.S. troops and to consult with top Bulgarian government officials.   ((AP Photo/Robert Burns))

Asked if he anticipated Russians escalating to deescalate, potentially using nuclear weapons, Austin said "any rhetoric concerning a use of a nuclear weapon is very dangerous," but asserted he was confident in the United States and NATO’s capabilities and ability to defend themselves. 

The Biden administration has warned the Russians might resort to using chemical or biological weapons in Ukraine and doing so would constitute a war crime. Meanwhile, Putin recently delivered a chilling speech claiming the U.S. backed laboratories in Ukraine by the Russian border are creating bioweapons – something the U.S. has categorically denied – and now China is repeating Moscow’s talking point. 

"I think if a chemical or biological weapon was used, you would see a significant reaction from not only the United States, but also the global community," Austin said. "I think what we've seen from the Russians in the past is they're 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 or us, NATO."

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In the last two weeks, the U.S. has already provided over $300 million worth of equipment to Ukraine, and the $800 million recently signed off on by President Biden will bring the total to over to $2 billion in terms of the amount of security force assistance, Austin said. The U.S. and allied forces have been training Ukrainian troops since 2014 when Russia annexed Crimea, so Austin stressed, "not only have we provided them equipment, but they were ready to use that equipment once we provided." 

The Associated Press contributed to this report.