The White House has confirmed that Ukraine is using US cluster bombs against Russian forces in the country.
National Security Spokesman John Kirby said initial feedback suggested they were being used "effectively" on Russian defensive positions and operations.
Cluster bombs scatter multiple bomblets and are banned by more than 100 states due to their threat to civilians.
The US agreed to supply them to boost Ukrainian ammunition supplies.
Ukraine has promised the bombs will only be used to dislodge concentrations of Russian enemy soldiers.
"They are using them appropriately," Mr Kirby said. "They're using them effectively and they are actually having an impact on Russia's defensive formations and Russia's defensive manoeuvring. I think I can leave it at that."
The US decided to send cluster bombs after Ukraine warned that it was running out of ammunition during its summer counter-offensive, which has been slower and more costly than many had hoped.
President Joe Biden called the decision "very difficult", while its allies the UK, Canada, New Zealand and Spain opposed their use.
The vast majority sent are artillery shells with a lower than 2.35% "dud rate", a reference to the percentage of bomblets which do not explode immediately and can remain a threat for years.
The weapons are effective when used against troops in trenches and fortified positions, as they render large areas too dangerous to move around in until cleared.
Russia has used similar cluster bombs in Ukraine since it launched its full-scale invasion last year, including in civilian areas.
Reacting to the US decision to send the bombs, Russian President Vladimir Putin said his country had similar weapons and they would be used "if they are used against us".
Oleksandr Syrskyi, the Ukrainian general in charge of operations in the country's east, told the BBC last week that his forces needed the weapons to "inflict maximum damage on enemy infantry".
"We'd like to get very fast results, but in reality it's practically impossible. The more infantry who die here, the more their relatives back in Russia will ask their government 'why?'"
He added however that cluster bombs would not "solve all our problems".
He also acknowledged that their use was controversial, but added: "If the Russians didn't use them, perhaps conscience would not allow us to do it too."
We’re arming Ukraine to preserve the rule based order established after War II that dictates sovereign nations cannot have their territory unilaterally annexed by another nation. Allowing this to happen without support to Ukraine would only tell other despots looking to start wars of conquest that they are allowed to do so without repercussion.
I'm on board with that basic premise, but when we start arming them with cluster bombs? It seems like we need to at least pump the breaks and check what our goals are and where we're willing to go. What about nerve gas? Land minds? Aerial war in the Russian mainland? I think even the people who disagree with me surely have some boundaries that they're not comfortable with. This seems like a point for use each to inquire how much devastation is productive for the people who actually bear the brunt of these choices.
I mean, these cluster bombs WILL kill innocent civilians after the war. Are they on board with that? I highly doubt they've signed off on this particularly strategy.
The West has already armed Ukraine with landmines. Western mines typically deactivate after a set amount of time. Nerve gas isn’t a useful weapon because you can’t control the wind. You’re positing a slippery slope, which is a fallacy as at every step of the way we are able to say “no, that’s not a good idea”. As for who is dealing with the brunt of the suffering from this war, the Ukrainians, polls indicate a supermajority of support for Zelenskyy. His government has indicated that they cannot negotiate with Russia at this time as they do not negotiate in good faith. They want Russia in a position where they’re incapable of violating any agreements they make rather than another situation like the Minsk Accords where Russia just bided their time and invaded even more extensively.