This was announced on March 3 by Deputy Secretary of State Victoria Nuland on the Youtube channel of the Echo of Moscow radio station.
"We are concerned that Russia is using cluster bombs in areas where civilians live," Nuland said.
Asked if the United States had evidence of Russia's use of illegal weapons in Ukraine, Nuland said: "We have evidence that cluster bombs were used, we have a wide range of information that there is also an intention to use other weapons."
A cluster bomb is a projectile that is dropped from the air or launched from the ground.
Such ammunition explodes in the air, releasing dozens and sometimes hundreds of smaller bombs capable of covering an area the size of several football fields.
This means that everyone in the area, including civilians, is at risk of death or injury.
Sometimes such bombs do not explode immediately, allowing them to kill or maim people long after the conflict is over.
The victims of cluster bombs are often civilians, including children.