"We just got them, we haven't used them yet, but they can radically change (the battlefield)," Ukrainian army commander Oleksandr Tarnavskyi told US broadcaster CNN.

The United States last week said it would deliver cluster munitions to Ukraine to aid its battle against invading Russian forces, despite concerns over the long-term risk posed to civilians by bomblets that fail to explode.

"The enemy also understands that with getting this ammunition, we will have an advantage," Tarnavskyi said, adding that Ukrainian forces would not deploy the weapon in heavily populated areas.

US Lieutenant General Douglas Sims later confirmed the news, saying the "cluster munitions are in Ukraine."

Asked about the slower-than-expected pace of advance in Ukraine's counteroffensive, he told journalists that "this is hard warfare, it's in really tough terrain, it's under fire".

"When you consider all that, it's pretty remarkable."

US President Joe Biden said the decision to provide cluster munitions was "very difficult" but stressed that Ukraine needed extra ammunition to refill its depleted stocks.

The controversial weapons are banned by numerous countries -- notably in Europe -- that are signatories to a 2008 Oslo Convention, to which neither Russia, the United States nor Ukraine are parties.

The Kremlin has said it will take "countermeasures" if the weapon is deployed by Ukraine against its troops.