The United Auto Workers (UAW) union has set a hard deadline to strike three Michigan General Motors factories if the two sides aren't able to agree on local labour pacts, the company said on Thursday.

According to the Detroit News, the union notified GM of its intention to issue five-day notices, which are required before a walkout, to the Flint truck assembly plant, the Delta Township assembly plant near Lansing and a Warren transmission plant.

If there are no local deals at the end of five days, the UAW could extend the deadline and continue negotiating or send workers off the line, the paper said. It was not clear exactly when that five-day period starts for each plant, but likely began either Thursday or today (11 April), depending on when the notices were delivered to GM, the Detroit News said.

The paper noted that the union last week warned GM that it may strike five plants, including the three in Michigan but the two sides have since reached a tentative agreement in Parma, Ohio. The fifth factory, an assembly plant in Arlington, Texas, was not included in the five-day warnings.

"Our energies remain focused on continuing the bargaining in good faith toward reaching tentative agreements as soon as possible," GM spokesman Dan Flores told the Detroit News.

The paper noted that analysts have questioned whether the local strike threats are a UAW ploy to draw GM into the American Axle dispute. Leaders at some of the union branches threatening to strike reportedly have said negotiators are struggling with issues including job guarantees for workers and plant work rules.