In breach of official guidelines, the United States military has fired thousands of rounds of depleted uranium (DU) while fighting Islamic State (Isis) in Syria.

Use of the toxic material falls within a legal grey area, but contradicts the notion that suffering to civilian populations should be minimised as much as possible.

Though US officials consistently denied use of the DU, Major Josh Jacques of US Central Command has now confirmed to Foreign Policy that the dangerous munitions have been used, in a report published Tuesday (14 February).

According to Jacques, 5,265 armour-piercing rounds containing DU were fired between 16 and 22 November 2015, in Syria's eastern desert.

DU is used because of its armour-piercing qualities and can be legally justified when used against armoured tanks.

However, the rounds were used to fire at IS (Daesh) oil trucks in Deir Ez-Zour, which were unarmoured and likely operated by civilians. It is also likely the radioactive chemical was not then disposed of effectively, causing further harm to the population.

DU is less radioactive than the original uranium, but is still a considerable health hazard causing birth defects and cancer following inhalation of the particles.

"The use of DU ammunition against oil tankers seems difficult to justify militarily on the basis of the arguments used by the US to support its use – that it is for destroying armoured targets," said Doug Weir, head of the International Coalition to Ban Uranium Weapons.

"Tankers are clearly not armoured, and the alternative non-DU HEI [high-explosive incendiary] rounds would likely have been sufficient for the task."

Jacques justified the US of DU because of a "higher probability of destruction for targets".

Legal scholars have argued greater clarity needs to be provided regarding the use of such weapons.

"I think this is an area of international humanitarian law that needs a lot more attention," said Cymie Payne, a legal scholar and professor of ecology at Rutgers University who has researched DU.

Prior to Syria, the US military also used DU in weapons during the 2003 invasion of Iraq. According to the Dutch peace group PAX, over 300,000 DU rounds were used during the course of the war, based on information it received from the Dutch ministry of defence.