DETROIT- Leaders of the United Auto Workers (UAW) turned up the pressure on General Motors Co on Sunday, ordering 850 maintenance workers at five GM facilities to walk off the job ahead of a meeting in Detroit on whether to call a wider strike.

After the union's four-year contract with GM expired without agreement on a new pact, the UAW said early Sunday that maintenance workers employed by Aramark would strike at five GM plants in Michigan and Ohio.

The UAW said the Aramark workers have been in negotiations since March 2018, and the company and union are at odds over pay and benefits. At GM's Flint, Michigan, assembly plant, which just launched a new heavy-duty pickup, UAW Local 598 leaders told members employed directly by GM they should report to work, crossing picket lines set up by fellow Local 598 members who work for Aramark. But union members could choose not to cross those lines.

The Flint heavy-duty pickup is vital to GM's profits. The UAW could call local strikes that target plants critical to production of large pickups and sport utility vehicles as an alternative to a company-wide strike that would more quickly drain union strike funds.

UAW leaders are expected to meet later this morning to decide what action to take and have called a press conference for 10 a.m. (1400 GMT) Sunday in Detroit.

UAW leaders signaled they remain far from a settlement with GM on a range of issues, including the future of four U.S. factories the company has indicated it will close.

"We still have many outstanding issues remaining, including significant differences (with GM)... on wages, health care benefits, temporary employees, job security and profit sharing," Terry Dittes, the UAW vice president who handles relations with GM, wrote in a letter to union officials issued late Saturday before the contract expired.

GM responded in a statement.

"We continue to work hard on solutions to some very difficult challenges," GM said. "We are prepared to negotiate around the clock."

On Friday, the UAW announced temporary contract extensions with Ford Motor Co F.N and Fiat Chrysler Automobiles NV (FCA) while it focused its attention on GM. The union had targeted GM as the first automaker with which it wanted to conclude contract talks. 

Sources briefed on the matter this week said GM may seek a temporary extension of the contract.

This year's talks had been expected to be contentious but have been complicated by recent developments in a longstanding federal investigation into corruption at the union. The probe has raised questions about UAW President Gary Jones, who was identified as an unnamed official mentioned in a searing federal complaint this week detailing alleged embezzlement by union leaders. 

Jones has not been charged and denies any wrongdoing. He remained in office after a meeting of union leaders Friday.

The spreading probe raises questions about the union's options and its leaders' standing with rank-and-file members. Last month, more than 96% of GM's hourly workers voted to authorize a strike if necessary, meaning if no deal is reached Jones could call for a walkout without further approval.

GM's workers last went out on a brief two-day strike in 2007 during contract talks. A more painful strike occurred in Flint, Michigan, in 1998, lasting 54 days and costing the No. 1 U.S. automaker more than $2 billion.

But the recent strike authorization vote was held before the dramatic events of the last few weeks, which included a late August FBI raid on Jones' home and other locations as part of the corruption probe. 

(Reporting by Nick Carey and Ben Klayman in Detroit; Editing by Daniel Wallis and Lisa Shumaker) ((nick.carey@thomsonreuters.com; +1 312 636 8837;))