UPDATE 1-GM says S.Korean unit will file for bankruptcy if no union concessions by April 20 - union

concessions by April 20 - union@ (Adds detail on Engle comments, background)

SEOUL, March 27 (Reuters) - General Motors said its loss-making South Korean operations would file for bankruptcy if its union did not make concessions by April 20, the union said on Tuesday.

The comments were made by Barry Engle, President of GM International, during a meeting with GM Korea's union leader on Monday, the union said in a document reviewed by Reuters.

The U.S. automaker announced in February it would shut down a factory in Gunsan, southwest of Seoul, and that it was mulling the fate of its three remaining plants in South Korea.

Engle said the South Korean government should also promise to provide support for GM Korea by April 20.

GM Korea, which needs $600 million in operational funds by the end of April, may consider more voluntary redundancies for the remaining 680 workers at the Gunsan factory, Engle said according to the union, adding that layoffs were the last option.

A GM Korea spokesman was not immediately available for comment.

The union said earlier this month that it will not demand a pay rise and bonuses this year, but instead wants the U.S. automaker to provide a future production plan and job security.

GM Korea still wants the union to agree to cut benefits, a union official said.

Earlier this month, GM Korea's second-largest shareholder, state-funded South Korean Development Bank ,began a due-diligence review of GM's South Korean unit as part of its decision whether to inject more capital into the money-losing operation.

GM Korea, which employs nearly 16,000 people, has previously said that without new funding from its major shareholders it would have a first-quarter "cash crisis". (Reporting by Hyunjoo Jin Editing by Lincoln Feast)