Apple has launched an investigation amid allegations that one of its suppliers was illegally employing students to make Apple Watches at a factory in Chongqing.

Hong Kong based labor rights group Sacom issued a report last week claiming Apple supplier Quanta Computer has been illegally using students on the Apple Watch production line at its factory in Chongqing.

"We are like robots"

Sacom says it interviewed 28 high school students at the Quanta Chongqing plant. The students said they were sent by their schools to the plant for "internships," yet did the same jobs of a production line worker, mostly irrelevant to their majors and often with illegal overtime and night shifts.

"We are like robots on the production lines. We repeat the same procedure hundreds or thousands of times every day, like a robot," said an 18-year-old student who works at the Apple Watch assembly line.

Many students said they were warned by their schools or teachers that if they refused to do the "internship" they would have problems receiving their graduation certificates.

The Sacom report suggests that at least half of production line workers are student in the Quanta Chongqing plant, with many of them aged 16 to 18.

"Basically, this department is composed of student workers from my school, as well as a few senior staff. At least three production lines in this department are operated by students from my school. There are around 120 students in the three lines and more in others," said the student.

"We are investigating"

Apple issued a statement on Monday, saying it has launched an investigation into these allegations from the Sacom report, according to CNN.

"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."

The world's most valuable company also says it has audited Quanta's factory in Chongqing three times between March and June and found "no student interns working on Apple products at that time."

According to Apple's Supplier Responsibility Standards, suppliers should ensure that all work performed by student workers is voluntary and relevant to their field of study.

Meantime, Quanta Computer has denied that it takes high school students on internships. The company says it is working closely with Apple on the probe.

"Not its first time"

The allegations echo another student labor scandal involving Apple last year in its Foxconn factory in Zhengzhou, where both Apple and Foxconn admitted student interns illegally worked overtime to assemble the iPhone X. The two companies vowed at that time to end the practice.

Sacom says it has been monitoring Apple's alleged practice of using student interns to replace regular workers in China since 2012.

The group claims it discovered an Apple plan to relocate its Watch production with illegal student labor to Quanta Computer's Chongqing factory last year, which has been denied by the two companies.

Sacom claims it conducted a further investigation into the issue this summer and found the existence of illegal student labor.

The Chinese government has been cracking down on illegal use of student and non-contract temporary laborers in recent years. According to the country's regulation on internships of vocational school students, students' internship should be related to their major and there should be no overtime work and night shifts during the internship.