Russia now has at least 50 percent of its massive invasion force inside Ukraine but is making slow progress due to unexpectedly stiff resistance, a senior US defense official said Saturday.

"We would estimate that more than 50 percent of the Russian force that Mr. Putin had arrayed against Ukraine ... has been committed inside Ukraine," the official told reporters.

"We have indications that the Russians are increasingly frustrated by their lack of momentum over the last 24 hours, particularly in the north parts of Ukraine," the official said, insisting on anonymity.

As of early Saturday Washington time, the Russian military had still not taken control of any Ukraine city, nor has it gained air superiority over the country, the official said.

Russian troops remained some 30 kilometers (18 miles) outside of Kyiv, the official said, while stressing that battlefield situations were actively changing.

Ukrainian resistance remains "viable," and Western allies are still able to get weapons and other supplies to them despite the Russian attack.

The Russians "have been frustrated by ... a very determined resistance and it has slowed them down."

"Based on what we've observed, that this resistance is greater than what the Russians expected," the official said