A labor group says data shows a higher than average injury rate at facilities operated by Amazon, which has defended its record on worker safety.

Amazon shifted policy on a controversial employee productivity monitoring system Tuesday as a coalition of US labor unions took aim at the firm, saying a need for speed in warehouses led to injuries.

Workers at Amazon warehouses are hurt more often and more severely than peers employed at retail rivals such as Walmart, the Strategic Organizing Center said in a report based on data provided to the Occupational Safety and Health Administration.

"The company's obsession with speed has come at a huge cost for Amazon's workforce," the center formed by said.

Late Tuesday the firm announced a shift in its Time Off Task policy—a heavily criticized system that uses algorithms to monitor workers' productivity, with employees made to explain their breaks.

If they cross a certain threshold, Bloomberg reported, they often face dismissal.

"Starting today, we're now averaging Time off Task over a longer period," Dave Clark, CEO of Amazon's global retail and logistics business, said in a blog post.

Amazon has invested heavily in workplace health and safety, implementing new technologies, processes and precautions to reduce the risk of injuries, spokesperson Kelly Nantel said earlier in reply to an AFP inquiry.

"While any incident is one too many, we are continuously learning and seeing improvements through ergonomics programs, guided exercises at employees' workstations, mechanical assistance equipment, workstation setup and design, and forklift telematics and guardrails—to name a few," Nantel said.

There were about six serious injuries per 100 Amazon workers last year requiring the employees to take time off or be shifted to light duties, according to OSHA data cited in the center's report.

That compares to a warehouse industry serious injury average of slightly more than three per 100 workers, the report indicated.

Founder Jeff Bezos told investors in April that the e-commerce giant needs a better "vision" for its workers, after an effort to create the company's first labor union was defeated.

Unions and political leaders have argued that Amazon employees face constant pressure and monitoring, with little job protection, highlighting the need for collective bargaining.

The news comes as the firm is poised to conduct Prime Day sales in late June that have become huge online shopping days due to deep discounts.

Online shopping has soared during the pandemic, with Amazon making a priority of getting orders to customers in just one or two days.

The Seattle-based tech and e-commerce powerhouse hired 500,000 people last year and now directly employs some 1.3 million people globally, according to Bezos.