US Defence Secretary Lloyd Austin has said he hopes Russian losses in Ukraine will deter its leadership from repeating its actions elsewhere, BBC reported.
Ukraine can still win the war if given the right support, he added.
He also announced the US would allocate an extra $713m (£559m) of military aid to Ukraine and other European nations.
Russian President Vladimir Putin has accused the West of attempting to "split Russian society and destroy Russia from within".
Mr Austin, a retired four-star general, was speaking after meeting Ukraine's President Volodymyr Zelensky in Kyiv. Also involving Secretary of State Antony Blinken, the visit marked the highest-level trip to Ukraine by US officials since the invasion began over two months ago, according to BBC.
The meeting between the US and Ukrainian parties, which ran for over three hours, comes as Russia escalates its military campaign in the south and east of the country.
At a news conference in Poland after the visit, Mr Austin told reporters the US wants to see "Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine".
The Pentagon chief added that US officials still believed Ukraine could win the conflict with "the right equipment" and the "right support".
BBC diplomatic correspondent James Landale observed that Mr Austin's comments calling for a weakened Russia were unusually strong for a US defence secretary. It is one thing to help Ukraine resist Russian aggression, it is quite another to speak of weakening Russia's capabilities, he said, BBC reported.
Mr Putin's accusations of the West attempting to "destroy Russia", made during a speech on Monday, appeared to be in response to Mr Austin's comments.
US defence officials said of the millions announced in fresh military funding, nearly $332m (£260m) would be allocated to Ukraine.
It brings the total US security assistance provided to Ukraine since the invasion began to more than $3.7bn (£2.9bn).
Mr Zelensky has been pleading with Western leaders to increase the flow of military equipment for weeks, vowing that his forces can overcome Russia's military if provided with fighter jets and other vehicles.
Last week the US confirmed it had supplied Ukrainian troops with howitzer artillery cannons and anti-artillery radars for the first time.
Russia's ambassador in Washington said Moscow had sent a diplomatic note demanding an end to the US supply of weapons to Ukraine.
Mr Blinken announced some US diplomatic staff would start to return to Ukraine from next week. They are expected to be based in Lviv initially, with a longer-term plan to re-open the US Embassy in the capital, Kyiv.
President Joe Biden intends to nominate Bridget Brink, a career diplomat, as US ambassador to Ukraine – a post that has remained vacant for more than two years, according to BBC.
Mr Blinken also defended America's diplomatic approach, telling reporters that the Western alliance the Biden administration had assembled had put pressure on President Putin's government.
"The strategy that we've put in place, massive support for Ukraine, massive pressure against Russia, solidarity with more than 30 countries engaged in these efforts, is having real results," Mr Blinken said. "And we're seeing that when it comes to Russia's war aims, Russia is failing, Ukraine is succeeding."
"A sovereign, independent Ukraine will be around a lot longer than Vladimir Putin's on the scene," Mr Blinken added, BBC reported.
Speaking following the meeting, Mr Zelensky said his government appreciated "the unprecedented assistance" from Washington and added that he "would like to thank President Biden personally and on behalf of the entire Ukrainian people for his leadership in supporting Ukraine".