Samsung spokesman James Chung said the company had noted a report by China Labor Watch that found workers at the plants were required to put in excessive overtime and could not sit down while they worked.
The watchdog's findings were based on investigations at eight factories six of which are directly run by Samsung and two by Chinese firms supplying parts to the technology giant.
"We admit what was pointed out about the overtime. We will take steps to re-evaluate our working hour practices," Chung said, adding that working hours were often extended when new product lines were built.
Chung said workers were required to stand at their stations due to the "basic structure of the assembly lines" and stressed that the same conditions applied in South Korean plants.
"We offer regular breaks for them. This issue is related to how the assembly lines were designed and is something we can't fix for a while," he said.
The China Labor Watch report contained a damning indictment of Samsung, saying the company forced employees at its China plants to work up to five times the legal overtime limit and denied them basic labour rights.
It said the transgressions amounted to "legal and inhumane violations" and stressed that workers lacked "any effective grievance channel" for rectifying the situation.
Chung dismissed the 'inhumane' tag as "totally unfair" and exaggerated.
"We agree with some of the criticisms raised, and that is why we said we would re-evaluate our working hour practices. But we do not agree with its overall characterisation of our working conditions," he said.
On Tuesday, Samsung had responded to a separate China Labor Watch report that alleged children under the age of 16 were employed at one of its Chinese suppliers, HEG Electronics in Huizhou.
Samsung said it would inspect its nearly 250 Chinese partners.
"We are implementing a rigorous plan to address any potential violations," the company said in a statement, vowing to terminate contracts with any Chinese suppliers in violation of labour norms.
The on-site inspections will be carried out by a 100-member team by the end of September, it said.
Initial audits conducted by Samsung found HEG employed teenaged student workers and interns, but none younger than 16, the company said.
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