"What we can confirm is that we know that for several days last week he was in the Donbas," a senior US defense official told journalists.

"We don't believe that he's still there -- that he has left and he's back in Russia," the official said.

"We cannot confirm reports that he was injured."

According to reports citing Ukraine officials, on Saturday Ukraine forces shelled a command center in Izium, where Gerasimov, chief of general staff of the Russian armed forces, visited to meet with top field commanders.

But he had apparently left the site before the shelling took place.

Gerasimov was believed to be touring the front to better understand field conditions and rally his troops as Russian forces, after failing to capture Kyiv in the north, attempt to take control of the Donetsk and Luhansk regions of the east.

In a daily briefing on the war, the Pentagon official said Monday that Russian forces were making minimal progress in a tough fight against Ukraine troops in the region and called their efforts "anemic."

"We continue to see minimal at best progress by the Russians in the Donbas. They've had some minor gains east of Izium and Popasna in the Luhansk Oblast."

The officials described the Russians as winning control of villages, then ceding them back to Ukraine fighters, and characterized Russian forces as demoralized and suffering from poor leadership and organization.

The Russian effort on the ground is "very tepid, very uneven... and in some cases, quite frankly, the best word to describe it would be anemic," the official said.

The official did not provide any similar assessment of Ukraine forces.

Arms deliveries from the Untied States and Western allies to Ukraine, including heavy artillery, radar systems, and attack drones, continue at a high pace, the official said.

Over the last 24 hours, two dozen flights carrying US arms landed near Ukraine, and another 11 were planned over the coming 24 hours, officials said.

In addition to that, 23 transport flights carrying arms and battlefield supplies from five other countries also landed in key delivery locations.