The United States used depleted-uranium antitank rounds twice during 2015 in air strikes against convoys of Islamic State (IS) tanker trucks, the Pentagon has said.
The military prizes depleted-uranium munitions for their armor-piercing capability in battles against tanks and armored vehicles.
But they have been criticized for posing health risks to soldiers who use them and being potentially toxic to surrounding civilian populations.
The United Nations Environment Program has described them as "chemically and radiologically toxic heavy metal," with about 60 percent of the radioactivity of natural uranium.
A military spokesman on February 16 said A-10 attack aircraft used depleted uranium rounds on November 16 and 22, 2015, in attacks on IS tanker trucks carrying oil, destroying hundreds of trucks.
Thousands of depleted-uranium rounds were fired in combination with other incendiary rounds "to ensure a higher probability of destruction of the truck fleet," U.S. Central Command spokesman Major Josh Jacques said.
Jacques added that depleted-uranium rounds remain an option for the future in battles against IS.
The munitions have been suspected -- but never proved -- to be a cause of the debilitating "Gulf War syndrome" suffered by veterans of the 1990-91 war.