Russian forces have made gradual headway in the region while launching missile and rocket attacks to curtail the movements of Ukrainian fighters elsewhere.
Mr Kahl estimated that Russian forces have sustained up to 80,000 deaths and injuries in the fighting, though he did not break down the figure with an estimate of forces killed.
He said the Russian troops have managed to gain “incremental” ground in eastern Ukraine, although not in recent weeks.
“But that has come at extraordinary cost to the Russian military because of how well the Ukrainian military has performed and all the assistance that the Ukrainian military has gotten.
And I think now, conditions in the east have essentially stabilised and the focus is really shifting to the south.”
The US and allies still are evaluating whether to supply aircraft to Ukraine, Kahl said. It’s “not inconceivable that western aircraft down the road could be part of the mix,” he said.