The United States used depleted uranium anti-tank rounds on two occasions in 2015 during devastating air strikes against convoys of Islamic State tanker trucks, the Pentagon said Thursday.

The military prizes depleted uranium munitions for their armor-piercing capabilities as well as for protective armor for tanks and 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."

A by-product of uranium enrichment, depleted uranium "is mildly radioactive, with about 60 percent of the activity of natural uranium," it says.

A military spokesman said A-10 attack aircraft used depleted uranium rounds on November 16 and 22, 2015 in attacks on tanker trucks carrying oil for the Islamic State group.

The operations destroyed hundreds of trucks.

A total of 5,265 depleted uranium rounds were fired in combination with other incendiary rounds, US Central Command spokesman Major Josh Jacques said.

The combination of armor-penetrating and high explosive incendiary munitions was used "to ensure a higher probability of destruction of the truck fleet ISIS was using to transport its illicit oil," he said, using an acronym for the Islamic State group.

"We will continue to look at all options during operational planning to defeat ISIS, this includes DU rounds," he added.

The munitions have been suspected -- but never proved -- to be a possible cause of "Gulf War syndrome," the name given to a collection of debilitating maladies suffered by veterans of the 1990-91 Gulf War.

The UN Environment Program has conducted studies and clean-ups of areas affected by use of the munitions in conflicts in the former Yugoslavia and Iraq.