"We do think that some of it has to do with their own sustainment and logistics," the official said.
"And we also think that just in general... the Russians themselves are regrouping and rethinking and trying to adjust to the challenges that they've had."
Six days after Moscow invaded its ex-Soviet neighbor, the official said a massive Russian convoy north of Kyiv is barely moving, but that the US believes they still intend to surround and capture the Ukraine capital, by siege tactics if necessary.
The US official said the Ukraine military continues to challenge the invasion force, and that the Russians have not gained control of the skies above the country.
Nor have the Russians succeeded in taking their first major target, the Ukraine's second largest city of Kharkiv in the northeast, where the heaviest fighting has taken place.
But in the south, the Russians have connected their forces along the coast from Crimea to the Russian border in the east, and have surrounded the city of Mariupol.
The Pentagon believes that the advance of the 150,000-strong combat force Russia has committed to invading Ukraine -- around 80 percent of which has so far entered the country -- has moved much more slowly than planned, and now faces supply shortages.
"In many cases, what we're seeing are columns that are literally out of gas," the defense official said. "Now they're starting to run out of food for their troops."
The official also said, but offered no evidence, that there were signs of morale problems in the Russian force, which makes use of a large number of conscript soldiers.
"Not all of them were apparently fully trained and prepared, or even aware that they were going to be sent into a combat operation," the official said.
"We have picked up independently on our own indications that morale is flagging in some of these units," the official said.