The US Department of Defence says Russian forces have achieved little progress in the past 24 to 36 hours.
In particular, a 60 km (40 mile) convoy of troops pushing south into the capital Kyiv has been slowed by Ukrainian fighters and "remains stalled", spokesman John Kirby said on Wednesday.
"We believe the Russians are regrouping themselves and reassessing the progress they have not made," he told reporters.
Kirby noted the invading troops are generally facing less resistance in the south of the country than in the north.
But he said the Pentagon could not confirm that the southern city of Kherson has fallen, as the Russians have claimed.
In spite of their slowed momentum, he cautioned, Russia retains additional power at its disposal that is yet to be used.
BBC live