HOUSTON, Dec 29 (Reuters) - The U.S. National Labor Relations Board on Wednesday impounded ballots that will decide whether the United Steelworkers (USW) continues to represent workers at an Exxon Mobil (XOM.N) oil refinery in southeast Texas. The NLRB said it sequestered the ballots while it reviews unfair labor practice complaints filed by the union representing workers at Exxon"s Beaumont, Texas, complex.The USW alleged that a worker lockout, after contract talks failed to produce a new agreement, was an attempt to break the union and that Exxon improperly aided a union-removal campaign. It may be several weeks "before we have the results or know what the next steps are," NLRB spokeswoman Kayla Blado told Reuters. Exxon expects the allegations will be dismissed once the investigation is completed, said company spokesperson Julie King. "Exxon Mobil remains confident that it has acted in accordance with the law at all times." The union charges "not only prevented our employees" voice from being heard but also denied them an opportunity to end the lockout," she said. USW International Representative Bryan Gross said the union has provided the NLRB with information and affidavits to support its stance. "We hope this lets the company know they need to get back to the bargaining table," Gross said. Exxon locked out union members from the 2,700-acre refinery and lubricant oil packaging plant on May 1. The facility produces fuels and Mobil 1 motor oil, and has continued to run with managers and replacement workers. The eight-month-long lockout will end if union-represented workers sign a petition to remove USW local 13-243 or accept the company"s last contract offer, Exxon said. Union members rejected the contract offer in October. read more The refinery has continued to operate with managers and supervisors from around the nation at its controls. Exxon has said the 369,024 barrel-per-day (bpd) refinery is operating at maximum capacity. Ousting the union would require a 50%-plus-one share of votes cast. Exxon proposed a contract with terms it said would allow the plant to be competitive in low-margin environments. The union has said it wants to keep provisions that give workers a say in job assignments. Negotiators have agreed to some parts of a possible contract, including a six-year term and pay increases that would match those to be set by national USW contract talks that begin in January. Workers were locked out at Beaumont because of a strike notice issued by Local 13-243 that could not allow for safe, continuous operations, Exxon has said. The union said Exxon was preparing to lock out workers before the strike notice, pointing to portable housing for replacement workers installed inside the Beaumont complex in the weeks prior to the lockout. read more During the lockout, the number of workers at the refinery represented by the USW has dwindled from 650 to about 580, as workers took jobs at other refineries or chemical plants in the region, people familiar with plant operations said.