U.S. President Donald Trump ordered meat-processing plants to stay open on Tuesday to protect the food supply in the United States, despite concerns about coronavirus outbreaks, drawing a backlash from unions that said at-risk workers required more protection.

With concerns about food shortages and supply chain disruptions, Trump issued an executive order using the Defense Production Act to mandate that the plants continue to function. The order is designed in part to give companies legal cover with more liability protection in case employees catch the virus as a result of having to go to work.

The world's biggest meat companies, including Smithfield Foods Inc., Cargill Inc., JBS USA and Tyson, have halted operations at about 20 slaughterhouses and processing plants in North America as workers fall ill, stoking global fears of a meat shortage.

In Canada, an Alberta meat-processing plant is now the site of the country's largest outbreak. More than 1,300 cases are linked to two plants, Cargill and JBS in southern Alberta, which together supply about two-thirds of Canada's beef.

• THE LATEST Coronavirus: What's happening in Canada and around the world April 28

John H. Tyson, chairman of Tyson Foods, said on Sunday that the food supply chain was "breaking" and warned of the potential for meat shortages.

Before issuing the executive order, Trump told reporters in the Oval Office that signing the order, "... will solve any liability problems," adding, "And we always work with the farmers. There's plenty of supply."

A senior administration official said the U.S. government would also provide guidance to minimize risk to workers who are especially vulnerable to the virus, such as encouraging older workers and those with other chronic health issues to stay home.

UFCW, the largest U.S. meat-packing union, demanded that the administration compel meat companies to provide "the highest level of protective equipment" to slaughterhouse workers and ensure daily coronavirus testing.

20 U.S. workers have died, says union

U.S. meat companies slaughtered an estimated 283,000 hogs on Tuesday, down about 43 per cent from before plants began shutting because of the pandemic, according to U.S. Department of Agriculture data. Processors slaughtered about 76,000 cattle, down about 38 per cent.

Critics of Trump's order made clear that plants were being shut down for a reason.

• 1 in 4 Alberta COVID-19 cases now tied to meat plant, as outbreak spreads to nearby First Nation

"When poultry plants shut down, it's for deep cleaning and to save workers' lives. If the administration had developed meaningful safety requirements early on as they should have and still must do, this would not even have become an issue," Stuart Appelbaum, president of the Retail, Wholesale and Department Store Union, said in a statement.

More than 6,500 meat- and food-processing workers in the U.S. have been infected with or exposed to the new coronavirus, and 20 have died, the UFCW said on Tuesday.