Glencore PLC hit back against claims of "slavery-like" working conditions at its cobalt-producing mines in the Democratic Republic of the Congo.

Switzerland-based IndustriALL Global Union recently released a report claiming workers described such conditions at some Glencore mines in the DRC.

"Workers described their treatment and conditions of employment as no less than slavery, comparable to Guantanamo Bay," IndustriALL said in the report.

Glencore denied the claims, according to a March 23 Bloomberg News report, stating it "strongly rejects the claim made by IndustriALL of slavery-like conditions at our operations in the DRC."

Four unions associated with IndustriALL are represented at Glencore's Katanga Mining Ltd. and Mutanda operations, according to Bloomberg. Katanga Mining's main asset is the Kamoto mine.

The spat between Glencore and IndustriALL comes amid growing scrutiny of cobalt supply as the metal's profile rises on the back of increasing battery demand. Cobalt is a critical component of lithium-ion batteries that power many of the electric cars now being produced.

The price of cobalt has been on a tear in the past year or so as battery manufacturers and automakers scramble to secure supply — dominated by DRC mines — amid a growing market for electric cars. The increasing need has raised the profile of the metal and also underscored the problems facing end-users that need to source it.

Meanwhile, the DRC government has taken advantage of its power over cobalt production by hiking royalties on the metal to 10%, among other recent changes made to the country's mining laws.