The Russian military is facing more resistance than Moscow had expected in its invasion of Ukraine, including its advance on Kyiv, according to a senior United States defense official.

"We do assess that there is greater resistance by the Ukrainians than the Russians expected," the senior U.S. defense official said, adding Ukraine's command and control of its military "remains intact."

The official noted that Russia also appears to have lost some of its momentum.

The U.S. official, who spoke on condition of anonymity, suggested a mixed picture for the Russian effort to press an offensive that Washington and Kyiv say is aimed at decapitating the government and installing a puppet regime.

"We do assess that there is greater resistance by the Ukrainians than the Russians expected," the senior defense official said, adding Ukraine's command and control of its military "remains intact."

"They are not moving on Kyiv as fast as what we believe they anticipated they would be able to do. That said, they continue to try to move on Kyiv."

Still, Russia has not yet mobilized 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.

Russia's military intervention in Ukraine entered its second day on Friday, with the latest reports indicating that Russian troops were heading toward the capital Kyiv from several directions.

Putin had ordered the military intervention on Thursday, just days after recognizing two separatist-held enclaves in eastern Ukraine.

He claimed that Moscow had no plan to occupy the neighboring country, but wanted to "demilitarize" and "denazify" Ukraine.

Ukrainian President Volodymyr Zelenskyy accused Russia of trying to install a puppet government and said Ukrainians will defend their country against Russian aggression.

Tensions started escalating late last year when Ukraine, the U.S. and its allies accused Russia of amassing nearly 150,000 troops on the border with Ukraine.

They claimed Russia was preparing to invade its western neighbor, allegations consistently rejected by Moscow.