“There’s a lot of fog in war but I think it’s safe to suggest that the Russians have probably taken 70 or 80,000 casualties in the less than six months. Now that is a combination of killed in action and wounded in action and that number might be a little lower, a little higher, but I think that’s kind of in the ballpark,” Mr Kahl said.
It comes as the US sanctioned a further $1bn in new military aid for Ukraine.
The aid includes additional rockets for the High Mobility Artillery Rocket Systems, or Himars, as well as thousands of artillery rounds, mortar systems, Javelins and other ammunition and equipment.
US officials say the Himars and artillery systems have been crucial in Ukraine’s ongoing fight to try to prevent Russia from taking more ground.
The latest announcement brings the total US security assistance committed to Ukraine by the Biden administration to roughly nine billion dollars since Russian troops invaded in late February.
“At every stage of this conflict, we have been focused on getting the Ukrainians what they need, depending on the evolving conditions on the battlefield,” Mr Kahl said in announcing the new weapons shipment.
Until now, the largest single security assistance package announcement was for $1bn on 15 June.
But that aid included $350m in presidential drawdown authority, and another $650m under the Ukraine Security Assistance Initiative.
Monday’s package allows the US to deliver weapons systems and other equipment more quickly since it takes them off the defence department shelves.
For the last four months of the war, Russia has concentrated on capturing the Donbas region of eastern Ukraine, where pro-Moscow separatists have controlled some territory as self-proclaimed republics for eight years.
Russian forces have made gradual headway in the region while launching missile and rocket attacks to curtail the movements of Ukrainian fighters elsewhere.
Meanwhile, both Ukraine and Russia have agreed to let UN inspectors visit the Zaporizhzhia nuclear facility amid fears of a second Chernobyl.
Additional reporting by agencies