A senior US Defense Department official has said Russia was aiming to encircle the Ukrainian capital of Kyiv in the coming days.

The official shared Washington's latest assessment of the Russian invasion on Monday. He said nearly 75 percent of Russian troops amassed along the border were now in Ukraine. He added that Russia had fired more than 380 missiles.

The senior defense official said the advance on Kyiv appeared to be Russia's "main line of effort." He said Russian troops were now roughly 25 kilometers north of the city, having advanced 5 kilometers over the past day.

The official said heavy fighting continued in and around the second largest city of Kharkiv, with Russian troops also attempting to advance on Mariupol in the east. But he said neither city had been taken yet.

The official also said Russia had not achieved air superiority over Ukraine, adding that "Ukrainian air defenses remain intact and viable, both in terms of aircraft and missile defense systems."

Some media reports have quoted US government sources as saying that Belarus is preparing to send troops across its border into Ukraine to support Russia.

The senior defense official denied the reports, saying US officials "have seen no indications that Belarusian troops are being readied to move into Ukraine," or "that they are, in fact, moving or are in Ukraine."

On Sunday, Russian President Vladimir Putin ordered his country's deterrence forces, including nuclear weapons, to be placed on high alert.

The official said Russian moves were being monitored closely, adding that he did not believe "we've seen anything specific" as a result of Putin's order.