AFL-CIO President Elizabeth Shuler called on Exxon Mobil Corp to end a four-month lockout of workers at its Beaumont, Texas, refinery because of "potential safety risks," according to a copy of a letter seen by Reuters.

In the letter addressed to a committee chair of Exxon's board of directors, Shuler said use of temporary workers to operate the refiner in place of the locked-out employees creates the risk of an industrial accident at the refinery.

Exxon did not reply to a request for comment about the letter.

Temporary workers lack the experience and training of permanent employees, Shuler said.

"We believe that the risk of an industrial accident at the Beaumont complex can best be mitigated by negotiating a mutually agreeable end to the lockout."

Shuler's letter was addressed to Susan Avery, chairperson of the Exxon board of directors public issues and contributions committee and dated Aug. 30, the same day Exxon rejected the last of three proposals made by United Steelworkers union (USW) local 12-243 last month to end the lockout.

"The USW has filed unfair labor practice charges with the National Labor Relations Board, alleging that Exxon Mobil illegally made unilateral changes to the continuing terms and conditions of employment and supported a union decertification campaign," Shuler said.

Exxon locked out the hourly employees at the refinery and adjoining lubricant oil plant on May 1 to avoid a strike as a 75-day labor peace period came to an end following the contract's expiration of the contract.

The USW has said the company's proposal requires its members to give up long-standing seniority and would create a separate contract for workers in the lube oil plant from that of workers in the refinery.

Exxon has said the proposal would give it flexibility to be profitable in low-margin environments. (Reporting by Erwin Seba Editing by Chris Reese and Grant McCool)