Russia has suffered some 600,000 casualties in its war with Ukraine — more than its losses in every conflict since World War II combined, according to U.S. officials. 

This September was the deadliest month of the entire war for Russia, a senior U.S. defense official told reporters on a call Wednesday. 

“Russian losses, again both killed and wounded in action, in just the first year of the war exceeded the total of all Soviet losses in any conflict since World War II combined,” the official said.

However, the steep casualties are not a “definitive metric” of success for Ukraine, the official warned. Ukraine has also suffered mass casualties, though the U.S. has not disclosed how many. 

Ukrainian President Volodymyr Zelenskyy said in February that some 31,000 troops had been killed. 

The U.K. Ministry of Defence put Russia’s daily casualty count at 1,271 in September, and said some 648,000 Russians had been killed or injured in the war. 

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“It’s kind of the Russian way of war where they continue to throw mass into the problem, and I think we’ll continue to see high losses,” the U.S. military official said.

South Korea warned earlier this week that North Korea was sending its forces to fight alongside the Russians. 

Russia has also lost two-thirds of its pre-war inventory of tanks to Ukraine, along with 32 medium-to-large naval vessels. 

Russian President Vladimir Putin is “trying to avoid a mass mobilization because of the effect that would have on Russia’s domestic population,” the official said. 

“At this point, he has been able to significantly increase the pay of these voluntary soldiers, and he has been able to continue to field those forces without doing a major mobilization.”

“And I think we’re just watching very closely how long that stance can actually be one that he can maintain, and I think it’s an important one for all of us to watch very closely,” the official added.

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Ukraine’s military said it struck a base in southern Russia’s Krasnodar region storing nearly 400 strike drones on Wednesday.

Russia has made some progress in the Donetsk region, taking the town of Vuhledar earlier this month and pressing toward Povrosk, a key railroad hub and supply station for Ukraine. 

The U.S. official said the Russian strategy around Vuhledar and Povrosk had brought “substantial casualties” for minor gains.

Russia’s Kursk region, which Ukraine invaded in August, is also in the midst of heavy fighting. Ukraine had hoped to divert Russian troops from the front line to defend Kursk. Russia has since recaptured some of the region, though the military official said that Ukrainian troops could hold onto the Kursk region for months or longer. 

Meanwhile, the U.S. continues to pour billions into Ukraine’s defense. Last month, President Biden announced an $8 billion package for Kyiv to supply it with military equipment through January. It is the last of the $61 billion that Congress approved in April for Ukraine. 

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Despite Zelenskyy’s calls, Biden has resisted authorizing Ukraine’s use of U.S.-given long-range missiles, known as ATACMs, to strike inside Russia and take out its stores of weapons capabilities, for fear of escalation. 

Many U.S. lawmakers have backed Zelenskyy’s request, but the U.S. official said the Biden administration is not considering reversing its policy. He said many of the arms that Ukraine is looking to take out, like Russia’s deadly glide bombs, have been moved out of range of ATACMs.