Russian President Vladimir Putin threatened to retaliate with cluster bombs if Ukraine uses U.S.-supplied munitions against his forces.
Putin said in a video clip released by Russian state TV on Sunday issued a warning to Ukraine not to deploy the largely internationally outlawed cluster munitions that were shipped recently by the United States.
Cluster munitions are missiles and bombs that burst in the air over the target and scatter many small explosive devices.
Their use is controversial because a considerable proportion of them do not detonate but remain unexploded, like mines, endangering civilians long after hostilities have ended.
A total of 111 countries have outlawed cluster bombs under an international treaty, but the U.S., Ukraine and Russia have not signed that treaty.