Exxon Mobil Corp (XOM.N) has rejected a contract offer from the United Steelworkers (USW) local chapter representing hourly workers at the company's Beaumont, Texas, refinery and renewed its offer of a six-year contract, a union official said on Wednesday.

Exxon has been campaigning for a longer agreement with the Beaumont workforce, offering first a five-year agreement and then a six-year pact.

Sources have told Reuters Exxon wants a longer pact to avoid work stoppages if it expands the 344,600-barrel-per-day-capacity Beaumont refinery into the nation's largest, possibly reaching 850,000 bpd by the end of the decade.

On March 17, USW local 13-243 had offered the current four-year national refinery and chemical plant workers contract plus the next contract to be negotiated in 2019 without a work stoppage. The agreement would cover at least seven years.

Exxon put the six-year proposal on the negotiating table again on Wednesday, said Richard Landry, USW international representative.

"It's not about the construction, it's about taking us off the pattern," Landry said.

The pattern refers to national agreements. Going off pattern is seen as reducing the negotiating strength of USW locals with corporations like Exxon.

Exxon has told the union the offer will remain on the table through April 19. Previous similar proposals have been rejected by the USW.

The contract for USW members at the Beaumont refinery expired on Feb. 1, but workers have remained on their jobs on a 24-hour rolling extension of the previous contract.

The current proposal contains a $4,500 per member ratification bonus. It requires also 90-day notice of a strike or lockout that can only be exercised after the contract expires.

If accepted, the contract would offer the annual pay raises in the current four-year national contract and would follow the annual pay raises in the 2019 national agreement.

An Exxon representative was not immediately available to discuss negotiations with the USW at the Beaumont refinery.