Ukrainian officials have accused Russia of using cluster munitions, accusations the Kremlin has denied.
Such weapons are designed to target enemy troops and weapons over a broad area, and their use in populated areas inevitably would lead to mass casualties among civilians.
Cluster bombs, rockets and artillery shells open in the air, releasing submunitions, or "bomblets," that are dispersed over a large area and simultaneously hit multiple targets.
Beyond the initial impact, bomblets have a high rate of failure to explode, posing a long-time threat of killing and maiming people for a long time after they were fired.
Thermobaric weapons consist of a fuel container and two separate explosive charges, with the first detonating to disperse the fuel particles and the second igniting the dispersed fuel and oxygen in the air, creating a blast wave of extreme pressure and heat that creates a partial vacuum in an enclosed space. That makes the weapon particularly deadly for people in an enclosed space.
The Pentagon has said that Russian mobile launchers for thermobaric weapons were spotted inside Ukraine, but couldn't confirm their use.
Activists and international delegations stand next to cluster bomb units, during a visit to a Lebanese military base in the southern town of Nabatiyeh, Lebanon, Monday Sept. 12, 2011.