The United Auto Workers called a fresh strike at a General Motors factory in Tennessee, a surprise walkout after negotiators had been working nearly around the clock to finalize a new contract this weekend.

Workers at GM’s factory in Spring Hill, Tenn., were ordered to go on strike Saturday evening, according to people with knowledge of the union’s plans. The strike came just as the UAW confirmed that it reached a tentative agreement with Chrysler parent Stellantis on a new labor contract.