By Erwin Seba

HOUSTON, Jan 31 (Reuters) - Negotiators for the United Steelworkers union (USW) and Marathon Petroleum MPC.N continued to meet on Saturday, just hours ahead of a deadline for possible strikes at multiple U.S. refineries and chemical plants, the union said.

Late on Saturday afternoon, the union rejected an offer from Marathon, the lead negotiator for 26 U.S. refiners and chemical companies including Exxon Mobil XOM.N, Chevron CVX.N and Valero Energy. The offer would have provided a 13% increase in wages over a four-year contract, according to sources familiar with the talks.

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

A USW spokesperson said the union had no immediate comment on negotiations.

The pay increases in Marathon's proposal would have been for 3% in each of the first two years and 3.5% in each of the final two years of the agreement.

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

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."

The current four-year contract expires at 12:01 a.m. on Sunday, but that does not mean a strike will automatically begin at that time.

In past negotiations, the union has granted rolling 24-hour contract extensions to hammer out an agreement past expiration.

Workers will only walk off their jobs at plants where the union authorizes a strike.

At the start of the last nationwide strike in 2015, the USW called out workers at 11 refineries across the United States that employed 5,200 USW members. The refineries continued operating with temporary replacement workers.

The negotiations between the USW and Marathon 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; Editing by Nathan Crooks and Nia Williams)

((erwin.seba@thomsonreuters.com; +1 832 746 4269; Reuters Messaging: erwin.seba.thomsonreuters.com@reuters.net))