(CNN) US Defense Secretary Lloyd Austin on Friday said that the US has "seen a number of missteps" by Russia in its invasion of Ukraine.
In offering an assessment of Russia's troops, Austin told CNN's Don Lemon in an exclusive interview, conducted in the Bulgarian capital of Sofia, that the Russians "have not progressed as far as quickly as they would have liked."
Austin told CNN that the Russians have "struggled with logistics" and that he has not seen evidence of "good employment of tactical intelligence," nor "integration of air capability with a ground maneuver."
"There are a number of things that we would expect to have seen that we just haven't seen. And the Russians really have had some -- has presented us some problems. So, many of their assumptions have not proven to be true as they entered this fight," he said.
Since the invasion Ukraine began nearly a month ago, Russian troops have bombarded and destroyed large parts of cities, including Mariupol and Kharkiv, but Ukraine has been able to prevent Russia taking large swathes of the country, including the capital of Kyiv.
"I think (Russia) envisioned that they would move rapidly and very quickly seize the capital city. They've not been able to do that," the Pentagon chief said.
In the face of fierce Ukrainian resistance to its invasion, Russia is also coping with low troop morale and struggling to resupply the thousands of troops in the country, US and NATO officials told CNN this week.
Austin was also asked about China's position on the war.
Russia has also requested military support and economic assistance support from China, two US officials told CNN this week.
China has conveyed some openness to offering help to Russia, according to a US diplomatic cable to allies. Both Russia and Beijing have denied that there have been any such requests.
On Friday, President Joe Biden sought to dissuade China's President Xi Jinping from assisting Russia, warning his Chinese counterpart during a 110-minute long video call of the "implications and consequences" for Beijing if it were to provide material support to Moscow.
Austin told CNN that it's "hard to say" what China will do and declined to speculate.
"I would hope that China would not support this despicable act by Putin, I would hope that they would recognize a need to respect sovereign territory," he told CNN, adding that, "We've been clear that if they do that, we think that's a bad choice."
Meanwhile, the US has supplied Ukraine with financial and military aid in its fight against Russia, including an additional $800 million in security assistance that Biden announced this week, but has stopped short of helping operate a no-fly zone over Ukraine as it seeks to avoid being directly involved in the conflict.
Austin told CNN that Biden has "been very clear about the fact that we won't have troops engaged in combat with Russia in Ukraine."
The defense secretary said that if the US were to put into place a no-fly zone, it would require controlling the skies, engaging Russian aircraft, and taking out aircraft systems in Ukraine, Belarus and Russia.
"So that would mean that we're in combat with Russia. These are two nuclear powered countries that nobody wants to see engage in the conflict. It's not good for the region. It's not good for the world," he said.
He said the US is focused on providing the type of assistance to Ukraine that "we know are making a difference" and doing "everything within our power to defend every inch of NATO territory."
In a news conference Saturday in Sofia, Bulgaria, Austin said that Russia continues to make "incremental gains" in Ukraine's south and that Russia has used "brutal, savage techniques'' in the way it has targeted civilians in Ukraine.
"Putin's war of choice against Ukraine has been tragic. Russia's invasion has taken a terrible toll on Ukrainian lives, including brave soldiers and far too many innocent civilians. Yet, Russia's aggression has galvanized the Ukrainian people, NATO, and the free world," Austin said in a joint news conference with Bulgarian Prime Minister Kiril Petkov.
Austin traveled to Europe this week, stopping in Belgium for the NATO Defense Ministerial before visiting Slovakia and Bulgaria as part of a trip to bolster NATO allies.
This story has been updated with additional developments and reaction.