DETROIT — General Motors intends to start calling in workers to its U.S. factories starting Monday to help prepare facilities to restart vehicle production amid the coronavirus pandemic.

But GM has not yet given a definitive date to resume production since agreeing to idle all its U.S. assembly plants in mid-March. Ford Motor Co. and Fiat Chrysler Automobiles also idled their U.S. plants, all in an effort to protect workers from the coronavirus. FCA has said it intends to restart production beginning May 4.

In an alert GM sent to employees, it said it will need some workers, mostly salaried, to come back to the factories to help ready the plants.

Any skilled trades workers that GM asks to come in will do so on a volunteer-only basis, said the person.

In response to GM’s employee alert Thursday, UAW spokesman Brian Rothenberg said, when it comes to recalling workers, “UAW President Rory Gamble has said the single focus should be on the health and safety of members and the litmus test should be whether your own family member would be safe going into that plant.”

Gamble has since said that early May is too soon to safely send workers back into the plants, noting there is not the scientific data to conclusively show that it is safe to have members back in the workplace.

In response, GM spokesman Jim Cain said: “When people do return to work, whether for planning or for regular production, we will be screening, cleaning and social strategies at all our facilities that were designed using the best medical and scientific data available, including guidance from the CDC.”