Russian forces are continuing to face logistical problems such as a lack of correct equipment, which is leading to frostbite among soldiers, a US senior defence official has said.
In a briefing, which the BBC has seen via its US partner CBS News, the US Department of Defense official also said there are signs that Ukrainians are now able to take back some territory the Russians have taken, such as in Izyum and Kherson.
The Russians are likely to pull battalion tactical groups deployed to other countries back to help in Ukraine.
The Defense Department has seen clear evidence over the last week of Russians deliberately targeting civilian infrastructure. It also believes there are signs of on-the-ground behaviour that would likely constitute war crimes.
Russia has likely been firing into the besieged south-eastern city Mariupol from the Sea of Azov.