“As part of our pledge against child labor, Samsung routinely conducts inspections to monitor our suppliers to ensure they follow our commitment,” Samsung said in a statement. “We are urgently looking into the latest allegations and will take appropriate measures in accordance with our policies to prevent any cases of child labor in our suppliers.”

The situation at the factory in Dongguan underscores some of the challenges multinational corporations face in sourcing goods from here. Wages and working conditions in China have steadily improved over the last decade. But ensuring that supplier factories comply with guidelines set by global brands, as well as China’s labor laws, is difficult, even though larger factories are regularly audited by outside inspectors.

Many global brands have struggled with labor problems in their Chinese operations. In the last few years, Apple has come under scrutiny in China over labor and safety problems, notably a spate of worker suicides and unrest at facilities run by its biggest contract manufacturer, the Taiwanese company Foxconn.

Apple declined to comment for this article, but the company has said it has taken steps to address labor issues in its supply chain, including deeper audits on its partners and a program that punishes suppliers that hire under-age workers.

Now, Samsung — whose smartphones are popular worldwide — is also the target of labor rights activists. In a report released on Thursday, China Labor Watch, which is based in New York, accused Samsung of allowing a supplier in Dongguan to hire under-age workers, cheating those workers on pay, denying them overtime wages and failing to give them government-mandated labor contracts.

“After allegedly inspecting hundreds of Chinese suppliers, Samsung did not find one child worker,” China Labor Watch said in a statement released on Thursday. “Yet in just one Samsung supplier factory, C.L.W. has uncovered several children employed without labor contracts, working 11 hours per day and only being paid for 10 of those hours.”

For the last decade, labor rights groups have tried to draw attention to labor abuse and health and safety violations in some of China’s biggest factories. They often send young activists to work undercover in the workshops, document conditions, secretly interview workers and examine their pay stubs and employment contracts.