(CNN) — Negotiators for the United Auto Workers union and General Motors returned to the table Thursday morning, as rank and file workers from the automaker’s manned picket lines for a fourth day.

The strike by nearly 50,000 hourly workers started early Monday, spread across 31 factories and 21 other facilities in nine states primarily in the center of the country. Spokespeople for both the automaker and the union wouldn’t comment on the state of talks, other than to say they are ongoing. The union is on record as saying that despite progress there are still significant differences between the two sides.

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Beyond the question of wages and benefits, major issues include GM’s use of temporary workers and the union’s demand that four US factories slated for closure have new products allocated to them so they can reopen, or remain open. GM says it has offered to invest $7 billion in its US factories during the four-year life of the deal in order to create or preserve 5,400 jobs.

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The strike has shut down the nation’s largest automaker and reports are that it is starting to be felt at suppliers across the country. GM said it buys goods and services from about 10,000 US companies, from auto parts used on cars to supplies it needs in its offices and factories.

But the company is not currently accepting deliveries from its suppliers, who generally deliver goods on a just-in-time basis, shortly before they are put onto vehicles along the assembly lines. Experts says the strike could eventually lead to a ripple effect with hundreds of thousands of US workers being laid off as the impact of the strike spreads.

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