Read also: The West should give Putin an ultimatum
Kirby was commenting on Russian missile strikes on Ukraine's civilian infrastructure a few days before the beginning of winter.
Read also: Putin says he should have seized Donbas in 2014
"And it's designed to want - for one reason, and that's to try to bring the Ukrainian people to their knees, because he can't bring the Ukrainian armed forces to its knees," he said.
The official emphasized the United States will continue to help the Ukrainian military, which is demonstrating success on the battlefield.
Read also: Rumors of Belarus joining Ukraine war is Russian intimidation, says Ukrainian intelligence official
"And so, what we're doing is a couple of things, Kirby said.
"First of all, we're still going to continue to provide the Ukrainian armed forces with the tools, weapons, the capabilities, the training that they need so that they can be successful on the battlefield. And they have been very successful in just the last few weeks."
Read also: Ukrainian intelligence does not confirm reports of mass imports of chemical weapons from Russia into war zone
Due to Russian missile strikes on Ukraine's critical infrastructure on Nov. 23, the power deficit in the energy system amounted to 27% as of early Nov. 28, with emergency power cuts throughout Ukraine.
Meanwhile, the Ukrainian military has warned about the threat of another Russian missile attack on Ukraine's critical infrastructure.