White House national security adviser Jake Sullivan said Sunday that Russian President Vladimir Putin's recent call-up of reservists into the military and referenda being carried out in occupied regions of Ukraine do not signal growing strength, but rather that the Russians are "struggling badly" in their ongoing war.

"What Putin has done is not exactly a sign of strength or confidence, frankly, it's a sign that they're struggling badly on the Russian side," Sullivan said in an interview with "Face the Nation." "And we're going to help the Ukrainians be able to take advantage of the gains they've made, and to continue to push back against the Russian forces that are brutally occupying portions of their country."

Putin announced a partial mobilization of his military reserves last week, a move that has prompted protests in Moscow and an exodus from Russia of fighting-age men fleeing to avoid being conscripted. The mobilization by Putin came two days before people in Russian-occupied regions of Ukraine began voting on whether to become part of Russia, a process that has been condemned by the White House and U.S. allies.

Sullivan said neither Putin's call-up nor the "sham" referendums will stop the U.S. from providing Ukraine with the "necessary means and technologies" needed for the country to defend itself from Russia in the ongoing war.

"I think what we are seeing are signs of unbelievable struggle among the Russians," he said. "You've got low morale, where the soldiers don't want to fight. And who can blame them because they want no part of Putin's war of conquest in their neighboring country. You've got Russia having depleted its stores of precision-guided munitions. You've got Russia disorganized and losing territory, to a capable Ukrainian force. And you've got a huge amount of infighting among the Russian military leadership, and now the blame game has started to include these replacements."

Sullivan said that while Russia is "struggling," it "remains a dangerous foe and capable of great brutality," as demonstrated by mass graves found in Izium, a city in northeastern Ukraine.

"We continue to take that threat seriously," he said.

Sullivan also pledged Ukraine will get more artillery, air defenses systems and tanks, and noted the U.S. has provided Ukraine with more than $15 billion in weapons and facilitated the transfer of tanks from NATO allies.

"We continue to see our obligation being providing Ukraine all that it needs to be able to effectively defend itself and defend its country and defend its freedom," Sullivan said. "That's what we're intent on doing. And we are not taking our eye off the ball."