A group that monitors working conditions in China has accused toymaker Mattel Inc. of multiple labor violations at Chinese factories.

China Labor Watch, a non-profit organization based in New York, said on Thursday that its investigators, some working undercover, documented 15 sets of violations. Those included illegal overtime pay, excessive working hours, forced labor and a lack of safety training at the facilities supplying Mattel, an El Segundo company considered the world’s largest toymaker.

“Based on this report, we can see that Mattel’s entire system has serious problems, and this reveals a lack of concern by Mattel for its production workers,” Li Qiang, director of China Labor Watch, said in a statement.

Mattel said it was investigating the allegations and regularly audits factories and suppliers. “We take the welfare of the people who make our products very seriously,” company spokesman Alan Hilowitz said in an email to the Business Journal. He said Mattel is a founding supporter of the International Council of Toy Industries’ CARE Process, which monitors compliance with the council’s business practices code.

According to the China Labor Watch report, Mattel owns one of the factories it cited and the others were suppliers. Mattel said it had not used one of the suppliers cited in the report since 2009.

Mattel has had past challenges in monitoring its Chinese supply chain. In 2007, it implemented several reforms after being forced to issue multiple recalls of Chinese-made toys tainted with lead paint.

Shares closed up 50 cents, or 1 percent, to $37.52 on the Nasdaq.