Two major commodities-trading firms have amassed much of the world's copper supplies in their warehouses, partly by paying to divert shipments away from other storage hubs, traders and analysts say.

This concentration of copper supplies has sparked concerns among industrial consumers of the metal. Some manufacturers and builders say they are worried that those stockpiles of copper—which is used in goods including automobiles, circuit boards and plumbing fixtures—could prove tough to procure if demand were to pick up sharply...