Hong Kong (CNN Business) Apple is investigating allegations that one of its suppliers illegally used high school students to assemble its watches at a factory in China.

Hong Kong-based labor rights group Students and Scholars Against Corporate Misbehavior (SACOM) said in a report last week that it found students being made to work on Apple Watch production lines at a plant run by Quanta Computer in the city of Chongqing.

The groupalleges that students aged 16 to 19 were forced by their schools to work at the factory through compulsory internships, placed in jobs unrelated to their field of study, often made to work excessive overtime, and in some cases put on overnight shifts. All four of those practices violate Chinese regulations and Apple's own standards , SACOM said.

The allegations highlight the challenges faced by Apple ( AAPL ), the world's most valuable company, in monitoring its extensive global supply chains.

Apple said in a statement Monday that it audited Quanta's Chongqing factory three times between March and June, and found "no student interns working on Apple products at that time."

"We are urgently investigating the report that student interns added in September are working overtime and night shifts," Apple said. "We have zero tolerance for failure to comply with our standards and we ensure swift action and appropriate remediation if we discover code violations."

Quanta Computer, which is headquartered in Taiwan, denied it takes school students on internships. It said in a statement Monday that it always follows its customers standards and is working closely with Apple on the investigation.

SACOM's report is based on interviews with more than two dozen teenagers who worked at the Chongqing factory this summer. The report withheld the names of the students, citing privacy concerns.

One 18-year-old is cited as saying they worked on an Apple Watch production line that had a daily target of 1,200 units. "We are like robots on the production lines. We repeat the same procedure for hundreds and thousands of times every day," the student said, according to the report.

Apple has come under fire for labor practices in its supply chain in the past, including the illegal use of student workers.

Last November, Apple and one of its biggest suppliers, Foxconn, admitted that student interns illegally worked excessive overtime at a Chinese factory to assemble the iPhone X. Apple said at the time that it sent staff to the factory to address the violations.

Foxconn told the Financial Times in November that "all work was voluntary and compensated appropriately, [but] the interns did work overtime in violation of our policy." It said it had taken "immediate action to ensure that no interns are carrying out any overtime work."