Nearly half of all recorded injuries in US warehouses last year occurred at Amazon, according to a report released Tuesday by a coalition of unions.

The e-commerce giant has boomed during the pandemic with soaring home delivery demand, but has also faced criticism over workers' conditions and its labor practices.

"Amazon employed one-third of all warehouse workers in the US, but it was responsible for nearly one-half (49 percent) of all injuries in the warehouse industry," according to the report by the Strategic Organizing Center (SOC).

The SOC report said US Amazon workers sustained more than 34,000 "serious injuries" on the job last year, a rate more than twice as high as that at warehouses not owned by the company.

Amazon acknowledged an increase in the number of injuries as tens of thousands of employees joined its workforce, but argued the rate at which its people got hurt had declined.

"Like other companies in the industry, we saw an increase in recordable injuries during this time from 2020 to 2021 as we trained so many new people," the company said.

"However, when you compare 2021 to 2019, our recordable injury rate declined more than 13 percent year over year," it added.

The coalition said it relies on data provided by Amazon to the US Occupational Safety and Health Administration -- the federal agency responsible for preventing workplace injuries.

A new report takes at the number of injuries at Amazon's warehouses
A new report takes at the number of injuries at Amazon's warehouses AFP / Nicholas Kamm

"After relaxing some of its discipline systems in the first months of the Covid-19 pandemic, Amazon reimplemented its monitoring systems and production pressures in late 2020, and its injury rates rose substantially," the SOC said.

Hiring at Amazon has spiked during the pandemic.

In the United States, the company has gone from some 700 sites in 2020 to more than 900 in 2021, and from more than 200,000 employees in 2017 to over 560,000 in 2021, according to the report.

In June 2021, Amazon changed working conditions, including longer breaks for its workers who prepare, ship and deliver packages.

That decision came after a previous damning SOC report, and an attempt to unionize at an Amazon warehouse in Alabama. That failed, but the campaign exposed what many employees described as the company's intense pace.

"We need a better vision for our employees' success," wrote Amazon founder Jeff Bezos in an annual letter to shareholders in 2020.

"We are going to be Earth's Best Employer and Earth's Safest Place to Work," he promised.

But "in stark contrast to Jeff Bezos' recent pledge... the injury rate at Amazon facilities increased by 20 percent between 2020 and 2021," the SOC said.

Amazon workers in New York have voted to launch the first US union at the e-commerce giant, an underdog upset against a company that has steadfastly opposed organized labor in its massive workforce.