Still, Russia has not yet mobilised the majority of its forces arrayed around Ukraine, the official said, assessing that just about one-third have now been "committed" to its offensive.
The official cited indications of a Russian amphibious assault west of Mariupol, "putting potentially thousands of naval infantry ashore there". The United States had also observed more than 200 total missile launches so far.
Although most of the Russian targeting has been against Ukrainian military installations, some of the missiles have landed on civilian residential areas, the official said.
Ukraine's military was putting up a fight, the official said.
"They are fighting for the country," the official said, noting that Russia had yet to establish control of the airspace above Ukraine or used the extent of its electronic warfare capabilities.
"In general, the Russians have lost a little bit of their momentum," the official said. The official said several hundred American citizens have left Ukraine over the past 24 hours.