(Reuters) -Volkswagen workers will go on extended strikes on Monday, stepping up a bitter dispute with management over layoffs and what would be the first factory closures on German soil for Europe's largest carmaker, a union statement said on Thursday.

The IG Metall union said workers would down tools for four hours at nine different sites in so-called "warning" strikes across the country, twice as long as the first round of industrial action at the start of December.

The walkouts are timed to coincide with the next round of talks between Volkswagen and labour representatives over cost-cutting measures that the company says are vital to stay competitive against rising costs and foreign rivals.

Volkswagen on Thursday said the management and workers representatives remained in dialogue to collaboratively develop viable, long-term solutions that balance economic stability and secure employment prospects.

The crisis at Europe's largest carmaker has hit Germany at a time of economic uncertainty and domestic political upheaval, as well as wider turmoil among the region's automakers.

Volkswagen Group CEO Oliver Blume on Wednesday had defended his decisions as necessary in a rapidly changing environment, saying management could not operate "in a fantasy world".

"It borders on mockery when Oliver Blume stands in front of the workforce and wishes them a Merry Christmas, while at the same time the VW board would prefer to put letters of termination under the Christmas tree for the employees," IG Metall negotiator Thorsten Groeger said on Thursday.

"We will now step up our efforts on December 9th and thus increase the pressure on the company at the negotiating table," he said in a statement.

Europe's car sector is in turmoil, with thousands of jobs on the line at automakers and their suppliers, all suffering from a weakening market on the continent and a slower-than-expected take-up of electric vehicles.

On Monday morning, thousands of VW employees are expected to attend a rally in Wolfsburg, where VW is headquartered, shortly before the start of negotiations.

Workers could increase pressure if no agreement is struck during next week's negotiations, union officials have signalled, leading to longer and possibly even open-ended strikes.