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Russia is setting up a "shaping operation" in preparation for a major ground offensive as it concentrates its war efforts in eastern Ukraine, a senior U.S. defense official told reporters Thursday. 

"They're setting the conditions for what they want to conduct in the Donbas region on the ground and in the air," the official said. "They are setting the conditions for what we believe will be a heavier offensive in the Donbas region."

Ukrainian multiple rocket launcher

A Ukrainian multiple rocket launcher BM-21 "Grad" shells a Russian troop position near Luhansk in the Donbas region on Sunday. (Anatolii Stepanov/AFP via Getty Images)

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The senior defense official said Russians are moving in artillery units, command and control "enablers," and aviation support. 

Over the last 24 hours, defense officials have seen helicopters being brought into the northern part of the Donbas region.

"They're doing the things that we believe [that] they believe they need to do to set the proper conditions for a renewed ground offensive," the senior defense official added. 

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Ukraine soldiers Russian tanks

Ukrainian soldiers walk next to destroyed Russian tanks in the outskirts of Kyiv, Ukraine, March 31, 2022. (AP Photo/Rodrigo Abd))

But the official also noted that western security groups have not seen a great influx of additional troops or tactical groups move into the region, despite previous warnings that Russia may be looking to recruit as many as 60,000 more troops for its ranks.

Russia is believed to have maintained roughly 80% of its fighting force since the beginning of its invasion and has largely removed its troops from western and central Ukraine as it hones in on the eastern front. 

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Ukraine soldiers

Ukrainian servicemen climb onto a fighting vehicle outside Kyiv, Ukraine, on April 2, 2022. (AP)

A senior defense official said Wednesday that roughly all of Russia’s ground forces are believed to be in the eastern Luhansk and Donetsk regions – an area referred to as the Donbas. 

"We don't believe that – while fighting is going on, clearly, in the Donbas – we don't believe their new offensive has begun, at least not in the terms that they are trying to define them as," a senior U.S. defense official told reporters Thursday. "It’s very difficult for us to tell you with certainty when D-Day is for this."