BERLIN (Reuters) -Germany's largest trade union, which sits on Volkswagen's supervisory board, said on Thursday that it will leave no stone unturned in coming up with alternatives to the carmaker's threats of plant closures, with a four-day week as one option.

The job security agreement put in place in 1994 was designed specifically to get the carmaker through difficult situations and should not be thrown overboard in difficult times, said Thorsten Groeger, head of IG Metall for the Lower Saxony region where Volkswagen is based.

Still, the union said it was impossible to lay out detailed proposals without more information on what solutions the company was proposing, said Christiane Benner, chair of IG Metall nationwide.

"We need forward-thinking ideas on where potential can be found," she said. "VW has survived difficult situations before."