General Motors (NYSE:GM), Ford Motor Company (NYSE:F), and Fiat Chrysler Automobiles (NYSE:FCAU) are aiming to resume some U.S. production on May 18, the Wall Street Journal reports. 

The Detroit automakers tentatively settled on the timeline after weeks of negotiations with leaders of the United Auto Workers (UAW) labor union and officials from Michigan Governor Gretchen Whitmer's office. 

Robots attach parts to a Ram 1500 pickup at FCA's Sterling Heights Assembly Plant in Michigan.

The Detroit automakers are eager to reopen their profitable truck factories, but the union still has concerns. Image source: Fiat Chrysler Automobiles.

The UAW's president Rory Gamble has expressed strong reservations about an early May restart. In a letter to members on Monday, he commended workers who have volunteered to go back to work early to prepare factories -- but emphasized that the union has not yet agreed to a date to restart. 

"Our concern here, first and foremost, is for the safety of our members and their families," Gamble wrote. "And that will be the determining factor in when we feel it is safe to reopen these facilities. We will not move on this point." [Emphasis added.]

The Wall Street Journal also reports that the automakers have been making progress in their efforts to draw up safety protocols that the UAW will accept, although the terms haven't yet been finalized.

Gov. Whitmer has extended an executive order closing the state's nonessential businesses until at least May 15. Her administration has not made clear whether auto factories are considered essential. The Detroit automakers, and most suppliers, voluntarily idled their Michigan plants in mid-March.

Separately, Ford said on Tuesday that it will begin reopening its plants in Europe on May 4, after it reached agreements with European labor leaders on safety protocols for those factories, including mandatory masks and daily temperature checks.