WASHINGTON ― The Pentagon’s international affairs chief said Monday that Russia’s new losses in the strategic southern Kherson region are about to spiral into a “major defeat” that would give Ukraine a defensive position amid “hot fighting” expected this winter.

After news that Ukrainian forces broke through Moscow’s defenses in Kherson, which Moscow saw as the gateway to Odessa, Assistant Defense Secretary for International Security Affairs Celeste Wallander said the Ukrainians were on the verge of pushing back the main Russian bridgehead across the Dnipro River.

“That would be a major defeat for Russia because it pushes back even more Russia’s ambition to take Odessa, which was one of the stated objectives earlier this year,” Wallander said at a Center for Strategic and International Studies event.

“It gives Ukraine another defensive position to ride out what probably will be hot fighting over the winter,” Wallander said, adding that in spite of Ukraine’s recent strategic successes, the U.S. and international community must continue to supply Ukraine with aid.

Already, Russia’s recent loss of the town Lyman, which its forces used as a logistics hub in Ukraine’s eastern Donetsk region, would “significantly affect Russia’s ability to supply, resupply and move its forces all along that forward line of conflict,” she said.

A senior U.S. military official said that Russian troops had fallen back to Kremmina, a town more than 30 kilometers east of Lyman. Denying Russia Lyman could impact its supply lines along the eastern front down to Bakhmut, a contested city 60 kilometers south of Lyman where the fighting continues.

Ukrainian President Volodymyr Zelenskyy announced Sunday that Russian forces were cleared from Lyman, one of the four areas in Ukraine that Russian President Vladimir Putin annexed last week.

Putin’s land grab has threatened to push the conflict to a dangerous new level. After U.S. President Joe Biden said last week the U.S. would never recognize Russia’s claims on those regions, Putin said he would consider the use of “all available means” to protect the territory ― which has been seen as a threat to use nuclear weapons.

Speaking Sunday on “Fareed Zakaria GPS,” U.S. Defense Secretary Lloyd Austin called Putin’s threats “an illegal claim” and “an irresponsible statement.”

“This nuclear saber-rattling is not the kind of thing we would expect to hear from leaders of large countries with capability,” Austin said, adding that, while he has warned his Russian counterpart about this path in the past, the decision is Putin’s.

Austin has not ruled out that Putin would act on his threats.

“There are no checks on Mr. Putin. Just as he made the irresponsible decision to invade Ukraine, you know, he could make another decision. But I don’t see anything right now that would lead me to believe that he has made such a decision,” Austin said.

Separately, Wallander said NATO should be asserting its unity and nuclear capability in answer to Russia’s “facile reliance on nuclear threats.”

Asked about the alliance’s plans since Russia invaded Ukraine seven months ago, Wallander hailed NATO’s new multinational battlegroups along its eastern border. She also said its members increasingly see spending targets “as a floor not a ceiling” and want to focus that spending on both modernization and ― with an eye on Russia’s hollow forces ― advanced training.

“I think the old debate about ‘is it deterrence by tripwire, deterrence by denial?’ is behind us now. It’s clear that what you need is an effective, credible defensive capability in order to deter an adversary, in this case, Russia,” Wallander said.

“What that credible defense looks like varies by areas of Europe. It includes not just ground forces, it includes air defense, it includes maritime domain, so it is multi-dimensional, but allies are really digging into what that looks like in their national capabilities,” she said.

Beyond billions in military, financial and humanitarian aid for Ukraine, Wallander said discussions within the U.S. about reconstruction aid are already underway. Wallander said the European Union, World Bank, International Monetary Fund should be involved.

“I think the main message is that it’s not the United States alone, it is actually these international institutions. And, and I think the Europeans have a strong commitment to supporting Ukraine,” Wallander said.

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

More In Pentagon
Biden drops out of 2024 race
Defense Secretary Lloyd Austin praised Biden for his "profound and personal commitment to the Department of Defense and the American military" on Sunday.