Moscow has said it has fired two hypersonic missiles in Ukraine, and while US Defense Secretary Lloyd Austin would not "confirm or dispute" whether Russia used such weapons, he warned that President Vladimir Putin's invasion was undergoing a change in tactics including the targeting of civilians.

Russia's use of the hard-to-intercept hypersonics would mark a dramatic escalation of its campaign to force Ukraine to abandon hopes of closer ties with the West.

But "I would not see it as a game changer," Austin told CBS talk show "Face the Nation."

"I think the reason he is resorting to using these types of weapons is because he is trying to re-establish some momentum," he added. "And again, we've seen him attack towns and cities and civilians outright (and) we expect to see that continue."

Ukraine's outgunned military has put up unexpectedly intense resistance that has slowed Russia's advance, stalling its forces outside the capital Kyiv and several other cities, making Moscow's supply lines vulnerable to Ukrainian attacks.

"They presented some significant problems for the Russians," and the stalling of Putin's troops on the ground "has had the effect of him moving his forces into a wood chipper," Austin said.

"The Ukrainians have continued to trip his forces, and they've been very effective using the equipment we provided them."

Austin visited Europe last week in a bid to tighten NATO unity and beef up the alliance's eastern flank.

US President Joe Biden has announced $800 million in new security assistance to Kyiv, answering Ukrainian President Volodymyr Zelensky's plea for more military aid and taking total US aid this month to $1 billion.

With Putin and Russia under punitive Western sanctions, Moscow has reportedly asked China for military and economic aid for its war, a claim Beijing denies.

As a potential means of reinforcing its troops, Russia has been recruiting thousands of Syrian army personnel and allied militia fighters for possible deployment in Ukraine, a war monitor has said.

"We've heard from a number of sources that this is, in fact, going on," Austin said, but "we have not seen mercenaries show up on the battlefield to my knowledge."