DETROIT (Reuters) – 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 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 working on an extended contract since March 2018, and the company and union are at odds over pay and benefits. UAW members employed by GM could choose not to cross picket lines.

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

The maneuvering over the contract talks comes after the U.S. Justice Department charged a top UAW official with embezzlement and linked UAW president Gary Jones to an ongoing corruption probe. Jones has not been charged and denies any wrongdoing. He remained in office after a meeting of union leaders Friday.

UAW leaders signalled 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 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 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.

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.

On Friday, the union’s executive board met but no changes were made to leadership, a union spokesman said.

The spreading probe raises fresh 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.

Sources briefed on the matter this week said GM may seek a temporary extension of the contract and pursue other options including seeking assistance from a third party.