UPDATE 3-GM says sticking to April 20 deadline for GM Korea restructuring

* GM in talks with S.Korea govt, KDB, union on restructuring unit

* Will seek bankruptcy protection if no common ground by deadline

* Tension with union has grown recently (Adds comments from KDB chairman)

DETROIT/SEOUL, April 13 (Reuters) - General Motors Co's president said common ground must be reached on a long-term financial restructuring of GM's South Korean auto unit by April 20 and if there was none, the operation would likely seek bankruptcy protection.

GM has been in talks since February, seeking support and incentives for the money-losing operation from the Korean government as well as concessions on wages from labor unions. The automaker plans to close one GM Korea plant and is weighing the fate of three others.

The April 20 deadline, first given in late March, ratchets up pressure on the South Korean government and the state-funded Korea Development Bank (KDB) which owns a 17 percent stake. The bank says it needs until early May to complete due diligence on GM Korea and the automaker's financial plan before committing public funds.

"Our preferred path remains to find a successful outcome here," GM President Dan Ammann told Reuters in an interview. "It's the right thing for all the stakeholders. But everybody has got to come to the table by next Friday."

An interim report on the due diligence will come out on April 20, KDB Chairman Lee Dong-geol told reporters on Friday, adding that meaningful progress in talks can be made after that.

While there were early signs that GM Korea's union may be more accommodating than in the past, tensions have grown.

Last week, some union members entered the office of GM Korea's CEO and destroyed furniture as the automaker was unable to make planned bonus payments. Talks with the union on Thursday were cancelled after the company demanded security cameras be set up at the site due to safety concerns.

GM headquarters this week also banned employee travel to GM Korea sites until further notice due to worries about security.

GM has offered to invest $2.8 billion in GM Korea, part of which would be shouldered by KDB. It is also seeking tax incentives and has proposed converting $2.7 billion debt owed to it by GM Korea into equity.

If GM Korea sought protection in South Korean bankruptcy courts, it could continue to build cars. However, it is not clear where the financing to continue operations would come from. The unit's creditors and employees also could take financial hits if GM Korea is restructured in court.

Complicating matters, GM Korea must make payments to workers who agreed to leave the company as part of a voluntary severance plan in the week beginning April 23. The unit does not have the cash to make the payments, and GM has so far indicated it will not provide any more cash unless there is a deal with unions and the South Korean government.

Quitting Korea would, however, not be an easy call for GM as it relies on those plants to be a key export hub, building vehicles for the United States and Latin America, although it could build models such as the Chevrolet Trax elsewhere.

GM also uses GM Korea to prepare what are known as "knock down" kits - partially assembled vehicles that are shipped to other markets for final assembly and it has strong relationships with South Korean suppliers that could suffer from a messy exit.

GM has abandoned several money-losing markets over the past three years as part of a broader strategy to boost profit margins and conserve capital to fund electric and automated vehicles and new models for core markets in China, the U.S. and Latin America. (Reporting by Joe White in DETROIT and Hyunjoo Jin in SEOUL; Editing by Edwina Gibbs)