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."
Sadly, the humanitarian concern of Ukraine's use of cluster bombs is not the most important.
The real issue is why we're sending Ukraine cluster bombs in the first place.
We're not doing it as part of some strategy or for some kind of tactical advantage. We're doing it because we don't have anything else left to send them. We've run out of modern munitions and won't be getting much more anytime soon. That's why we're sending Ukraine cluster bombs.
And the reason this article exists is because Ukraine is almost certainly not using them effectively, but they want to convince us otherwise. Cluster bombs don't do shit to tanks or buildings. They were designed for targeting people hiding in the jungle. Why would anyone feel the need to write an article saying, "Hey, btw, these weapons you paid for are definitely working out really well. They're super good at destroying Russians, for sure!"? I suspect it's because they're doing jack shit.
Cluster munitions certainly can take out tanks, but not all cluster munitions are made equally. There are several types. Just a quick Google search brings up the CBU-100 Rockeye II which is an anti-tank cluster munition.
We're also sending them these because cluster munitions are no longer used by the US military. So it's decomission or send them to be used. It's cheaper to ship them then dispose of them.
And we haven't ran out of modern munitions. We're running low on AVAILABLE SURPLUS munitions to send them. But we are ramping up production to replace current stocks.