SEOUL, March 26 (Reuters) - South Korea's central bank said on Monday it would grant 40 billion won ($37 million) of aid to mid-to-small sized companies that are hit by the shutdown of a General Motor's plant.

"There could be more aid if needed after monitoring the demand and overall impact on the local economy," said BOK in Monday's statement.

South Korea began doing due diligence on GM Korea to decide whether to extend financial support to the loss-making operations of GM, after the U.S. automaker announced a restructuring of the unit and a closure of one of the plants. ($1 = 1,080.5000 won) (Reporting by Dahee Kim; Editing by Simon Cameron-Moore)