By Erwin Seba

HOUSTON, Jan 31 (Reuters) - The United Steelworkers union said late on Saturday it would extend talks with Marathon Petroleum, averting a strike by 30,000 workers at U.S. refineries and chemical plants.

The rolling 24-hour extension means the current contract, which had been set to expire at 12:01 a.m. ET (0501 GMT) on Sunday, remains in effect until either the union gives a 24-hour notice to strike or Marathon issues a 24-hour notice to lock workers out.

Since negotiations began a little over a week ago, the union has rejected at least five proposals from Marathon. The last proposal on Saturday would have given refinery and chemical plant workers a 14% wage increase over a four-year contract.

Marathon is the lead negotiator for 26 U.S. refiners and chemical companies, including Exxon Mobil, Chevron and Valero Energy. The USW represents workers at plants that account for two-thirds of U.S. crude oil refining capacity.

Earlier on Saturday, Marathon spokesperson Jamal Kheiry said, "MPC continues to meet with representatives from the USW. We are committed to bargaining in good faith and to working toward a mutually satisfactory agreement."

Mike Smith, chairman of the Steelworkers' National Oil Bargaining Program, said union members were fighting for fairness and justice.

"Our unity and solidarity at every facility throughout the industry sends a clear message that we are prepared to fight for a fair contract," Smith said in a statement issued by the USW.

Sticking points for the talks are cost-of-living increases for the 30,000 oil industry workers represented by the union, healthcare costs and standards for use of artificial intelligence in the plants, said people familiar with the talks.

The USW is also pushing for tougher safety standards, but the sources said that seems to be a non-starter for Marathon.

"Marathon as a company thinks our industry is overpaid," said one of the sources, who asked not to be identified because they were not authorized to speak publicly. "They’re not coming up much on economics. And to be honest, they’re not really addressing anything else in our proposal besides AI. And they’re not addressing it in a good way."

In previous negotiations, the USW has granted 24-hour extensions for several days past a contract's expiration.

The negotiations are for a national pattern agreement that sets wages for the hourly union workers, healthcare costs, national agreements on safety and other issues.

Inside refinery operators make about $50 an hour after completing their probationary period.

The national agreement is combined with agreements on site-specific issues to create the contract for each plant.

Workers and the company settled local issues on Friday at Marathon's largest refinery, the 631,000 barrel-per-day Galveston Bay Refinery.

(Reporting by Erwin Seba, Additional reporting by Disha Mishra; Editing by Nathan Crooks, Nia Williams and William Mallard)