US Secretary of Defense Lloyd Austin told CNN on Thursday he has "no doubt" that Ukraine will become part of NATO after Russia's war against the country ends.

"I have no doubt that will happen, and we heard just about every country in the room say as much," Austin said in an interview with CNN's Wolf Blitzer in Vilnius, Lithuania, following a two-day summit that was dominated by the question of when Kyiv would join the alliance.

"I think that was reassuring to [Ukrainian President] Volodymyr Zelensky, there are other things that should happen, like judicial reform and things that make sure that the democracy is in good shape," Austin added.

NATO's summit in Vilnius began with tense exchanges between Kyiv and the West over the lack of a timeline for Ukraine's entry to the alliance, but ended Wednesday with a flurry of solidarity and security guarantees from Western leaders that Ukraine's future is with the military bloc.

When asked by Blitzer how close Ukraine were to meeting NATO standards, Austin said that there was "still work to be done."

"In terms of training and equipping, there is still work to be done," Austin said. "But we are doing this work now as they fight this war and so things have been done up to this point there is more that will need to be done to ensure that they have a full complement of capabilities."

US President Joe Biden had previously emphasized that Ukraine is not ready to enter NATO, telling CNN in an exclusive interview last week that Russia's war in Ukraine needs to end before the alliance can consider adding Kyiv to its ranks.

Biden acknowledged at the summit that the alliance did not invite Ukraine to membership as it works on "necessary reforms." But Biden added that "we're not waiting on that process to be finished" to boost the country's security.

During his CNN interview, Austin also emphasized that the US "don't seek a contentious relationship with China."

"We have reached out at multiple levels. It's really important for countries with our kinds of capabilities are able to talk to each other so that we can manage these relationships," Austin said.

"We don't seek a contentious relationship with China. Our relationship is one of competition ... As we continue to sail the international seas and fly the international skies, we want to make sure that everyone is doing things in a safe manner. We certainly are doing things in a safe and authorized manner."