Bernie Sanders has bared his teeth at Amazon again.

On Monday, he tweeted that Amazon needs to improve working conditions for its warehouse employees following a report from the Daily Beast detailing 911 calls.

Sanders previously demanded that Amazon raise its minimum wage, which it did in November last year.

He's not the only Democratic 2020 candidate to take aim at the tech giant.

Bernie Sanders has stepped back into the ring with Amazon.

The 2020 Democratic presidential candidate has been a vocal critic of the company and its CEO Jeff Bezos in the past. Previously he called for a $15 minimum wage, which Amazon introduced in November last year.

Sanders praised the decision at the time, saying: "Mr Bezos and Amazon are now leading the way."

Read more: Amazon warehouse employees speak out about the 'brutal' reality of working during the holidays, when 60-hour weeks are mandatory and ambulance calls are common

On Monday, however, Sanders turned up the heat on the company once more following a report from the Daily Beast detailing 911 calls from Amazon warehouses and workers experiencing serious mental health crises.

Sanders tweeted that Amazon "must recognize that workers' rights don't stop at the minimum wage." He added that the company must "significantly improve working conditions" and allow staff to unionize.

It follows a Business Insider investigation into warehouse working conditions last month, which uncovered the "brutal" reality of long hours, physical labor, fears about taking time off, workplace injuries, and the pressure to keep the wheels turning, even when the weather is treacherous.

Sanders isn't the only Democratic candidate to be upping the pressure on Amazon. Elizabeth Warren last week said that if made president, a keystone policy would be breaking up tech giants Amazon, Facebook, and Google's parent company Alphabet.