"I have no doubt that that will happen," Austin told CNN when asked if Ukraine will join the alliance after the war against invading Russian troops is concluded.

The US defense chief also noted much of Ukraine's inventory is still Soviet-era legacy equipment, saying that "in terms of training and equipping, there's work to be done" for the country meet NATO standards.

"We're doing that work as we're helping them as they fight this war," Austin said.

At a summit this week in Lithuania of the 31 NATO nations, Secretary General Jens Stoltenberg insisted the alliance had never used "stronger language" to back Ukraine in its bitter fight to defeat the Russian invasion.

NATO leaders vowed after the first day of the summit that "Ukraine's future is in NATO" and shortened the eventual process Kyiv would have to go through to enter the alliance.

But that did not go much beyond a 2008 vow on future membership, and reflects the concerns of dominant power the United States about being dragged into a potentially nuclear conflict with Russia.

The leaders of the G7 powers subsequently promised to support Ukraine for as long as it takes to defeat Russia's invasion.

Ukrainian President Volodymyr Zelensky said that amounted to a "significant security victory," but did not disguise the fact that he sought a firm timetable for Kyiv to join NATO.