"The Russians have struggled with logistics from the very beginning" of the Ukraine war and "are still struggling with logistics," Austin told journalists aboard a US military aircraft.

They are "experiencing significant shortages of artillery munitions," he said.

Kyiv has targeted Moscow's storage points, which caused "the Russians to begin to struggle a bit with the amount of ammunition that they have available."

Russian forces rely heavily on artillery, firing large numbers of rounds before they maneuver on the ground, Austin said.

"For that kind of operation, it requires a lot of munitions. I'm not sure that they have those kind of munitions to be able to support that going forward."

Austin also said Russia's supply of precision-guided munitions has been "significantly reduced" in the course of the nine-month conflict, and that Moscow will not be able to quickly replace them due to trade restrictions on items such as microchips.