HOUSTON, Feb 2 (Reuters) - The United Steelworkers union (USW) has neither accepted nor rejected the last offer from Marathon Petroleum ( MPC ) for a new four-year labor agreement for U.S. refineries and chemical plants, people familiar with the talks said ‌on Monday.

The offer, made in negotiations on Sunday, would give 30,000 workers represented by the ​USW a 15% pay increase over the length of the contract, ‍the sources said. 

Marathon is the lead negotiator for 26 ⁠companies including Exxon ⁠Mobil, Chevron and Valero Energy. Marathon is the largest U.S. refiner, operating 16% of ‌national capacity.

USW-represented workers operate sites that ​account for nearly two-thirds of U.S. refining capacity of 18.3 million barrels per day (bpd).

The contract agreed to by Marathon ⁠and the USW will also define ‍the ​contracts to be negotiated by other unions.

Neither Marathon nor the USW spokespeople immediately replied to requests for comment on Monday.

Marathon's last ‍proposal would provide a 4% increase in hourly wages in the first year, 3.5% pay hikes in each of the second and third years and another 4% boost in the fourth year.

The average pay for an inside operator at a refinery is about $50 an hour.

Agreement ​with ‍the terms would also get USW members a $2,500 signing bonus, with the remaining terms the same as previous contracts.

The union's ​National Oil Bargaining Program policy committee, which is delegated to approve any agreement, plans to meet the union members across the U.S. to discuss the Marathon offer.

The last proposal arrived hours after the current contract was set to expire at 12:01 a.m. on Sunday, but the Steelworkers agreed on Saturday to ​rolling 24-hour extensions of the current agreement until it gives notice of cancellation of the extensions and intent to strike, or Marathon gives notice of intent to lock ‍out workers.