(WASHINGTON) — The United States’ top military leaders said Monday that Russian President Vladimir Putin has opened “another front” in the Kharkiv region of Ukraine, home to the country’s second-largest city.

“Putin’s forces have opened another front to seize sovereign Ukrainian territory, and the Kremlin’s invaders are obliterating Ukrainian villages, killing innocent civilians and bombarding civilian infrastructure including dams and power plants,” Defense Secretary Lloyd Austin told reporters after a meeting of the Ukrainian Defense Contact Group, an international working group coordinating defensive assistance for Kyiv.

Despite the Russian advance, the defense secretary said U.S. weapons shouldn’t be used beyond Ukrainian territory.

“Our expectation is that they continue to use the weapons that we provided on targets inside of Ukraine,” Austin said.

U.S. military assistance, another $60 billion of which was passed by Congress and signed by President Joe Biden in April, is arriving as Ukraine faces a Russian offensive that could determine the “character” of the war, ​Can Kasapoğlu, a senior fellow and political-military affairs expert at the Hudson Institute, told ABC News.

Gen. C.Q. Brown, the chairman of the Joint Chiefs of Staff, said Monday that Russia’s new offensive “aim[s] to establish a shallow buffer zone along the Ukrainian border.”

“Russia anticipates that this will divert Ukrainian focus and capabilities from other critical areas,” he said.

Kharkiv was recaptured by Ukraine in a fall 2022 counteroffensive after Russia took the city in its initial invasion in February 2022.

Russia has not breached the Ukrainian front line, but Kasapoğlu said the front line is not stabilized, leaving doubt as to whether Ukraine can hold the city if Russia mounts an effort to take it.

“The Russians managed to secure many tactical gains” in the Kharkiv region and around the city of Kharkiv, Kasapoğlu said, and the Russians can be expected to “try to enhance these tactical footholds … and gradually move forward to get Kharkiv city in artillery range.”

“This may go beyond merely a subordinate effort or a distractive effort,” Kasapoğlu continued.

If the offensive is a main effort, and the Russians can recapture Kharkiv after winning it once and then losing it, “the chances are really slim for Ukraine to launch a large-scale counteroffensive and retake territory from the Russians,” Kasapoğlu said.

It could become clear “in the forthcoming weeks” whether Russia can “translate [its] tactical gains into strategic gains” and retake Kharkiv, Kasapoğlu said.

After completing its spring conscription, Russia has sufficient manpower, and the scope of the Kharkiv offensive is largely a function of whether Putin chooses to double down, Kasapoğlu said.

The United States’ supplemental package included much-needed artillery, as well as munitions for air defense, but Ukrainian President Volodymyr Zelenskyy said his forces need more.

The crisis in Kharkiv is “the world’s fault,” Zelenskyy told ABC News’ James Longman Friday, adding: “We cannot afford to lose Kharkiv.”

Air defenses, which Austin said the contact group discussed at length Monday, are crucial, Zelenskyy told Longman. 

“All we need are two Patriot systems,” he said.

The U.S. package includes munitions for the Patriots but not the systems themselves. The Germans have committed to providing one — a move praised by Austin on Monday — but the Pentagon chief said in April the system wouldn’t be a “silver bullet” for Ukraine’s defense.

Long-range ATACMS, a missile system the U.S. acknowledged it dispatched to Ukraine for the first time in April, could have made a difference in Ukraine’s early defense of Kharkiv, according to Kasapoğlu.

This would have been the “ideal weapon” to counter a heavy buildup of Russian troops, Kasapoğlu said, but because the Russians were striking for the first time from their own territory — and not from within Ukraine — Ukraine was restricted from using them by allies’ conditions.

Victoria Nuland, former undersecretary of state for political affairs, told ABC News’ Martha Raddatz on This Week Sunday that American weapons should be available to Ukraine for Russian targets.

“I think if the attacks are coming directly from over the line in Russia, that those bases ought to be fair game, whether they are where missiles are being launched from or where the troops are being supplied from,” she said.

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