(Reuters) - Amazon.com ( AMZN ) has been fined $5.9 million by a California labor regulator who says the online retail giant failed to properly inform workers of productivity quotas at two warehouses, including one where some workers are trying to unionize.

The office of California Labor Commissioner Lilia Garcia-Brower announced the fines, which were issued in May, on Tuesday.

A 2022 California law requires employers to provide written descriptions of quotas to workers if they can be disciplined for failing to complete jobs at a specified speed. The commissioner said Amazon ( AMZN ) violated that law nearly 60,000 times in a five-month period ending in March at massive warehouses in Moreno Valley and Redlands, outside of Los Angeles.

Amazon ( AMZN ) spokesperson Maureen Lynch Vogel said the company is appealing the citations and denied that warehouse workers have fixed quotas.

"At Amazon ( AMZN ), individual performance is evaluated over a long period of time, in relation to how the entire site's team is performing. Employees can - and are encouraged to - review their performance whenever they wish," Lynch Vogel said in a statement.

Criticisms of Amazon's ( AMZN ) alleged quota system have been a focal point of a nationwide campaign to unionize its warehouses. Workers at a New York City warehouse voted to join a union in 2022, while others at two facilities in New York and Alabama have since spurned unions.

A union in 2022 filed a petition to hold an election at the Moreno Valley warehouse, known as ONT8, which was later withdrawn amid allegations of illegal union-busting activity by Amazon ( AMZN ). An administrative judge is scheduled to hold a hearing on those claims, which Amazon ( AMZN ) has denied, in August.

Garcia-Brower in a statement said Amazon's ( AMZN ) quota system is exactly what the California law was designed to prevent.

"Undisclosed quotas expose workers to increased pressure to work faster and can lead to higher injury rates and other violations by forcing workers to skip breaks," she said.

Congress is considering a Democratic-backed bill that would largely mirror the California law by requiring written notice of quotas and prohibiting quotas that prevent workers from taking breaks or using bathrooms.

Senator Ed Markey of Massachusetts, one of the bill's sponsors, said the fines against Amazon ( AMZN ) announced on Tuesday highlighted the need to crack down on "punishing" quota systems.

"We need more than a patchwork of state laws," Markey said in a statement.