Russia is reportedly sending even more troops to its border with Ukraine, even after President Joe Biden warned Russian President Vladimir Putin that the U.S. would punish Russia and increase defensive assistance to Ukraine.

U.S. intelligence found Russia is sending additional military units to the border region, sources familiar with the intelligence told CNN on Friday. The latest U.S. intelligence revealed Russia has about 50 so-called “Battalion Tactical Groups” or BTGs — each consisting of about 900 troops — deployed on and around the Ukraine border. According to CNN’s sources, another six of these BTGs are now on their way to the Ukrainian border.

These Russian BTGs are meant to act as self-sustained fighting units, complete with their own infantry, artillery, anti-tank, reconnaissance and engineering units. These Russian units were heavily employed in the 2014 Russian annexation of Crimea.