A union representing tens of thousands of workers at Samsung Electronics in South Korea said Wednesday it would extend a three-day strike indefinitely in a bid to force management to negotiate.

The strike is the biggest labour action in the tech giant's history and steps up pressure on the chipmaker's management, who last week predicted a huge second-quarter operating profit increase.

"(We) declare a second indefinite general strike from July 10, after learning that the management has no willingness to talk," the National Samsung Electronics Union said in a statement.

More than 5,000 members stopped working Monday for what was meant to be a three-day strike, part of a long-running battle over pay and benefits.

The move follows a one-day walkout in June, the first such collective action at the company, which went decades without unionisation.

The union has more than 30,000 members -- more than a fifth of the company's total workforce.

Samsung told AFP Wednesday that the strike would not affect production.

"Samsung Electronics will ensure no disruptions occur in the production lines," a spokesperson told AFP.

"The company remains committed to engaging in good faith negotiations with the union."

But the union said it had confirmed "clear disruption in production," and added that the longer the strike went on "the more the management will suffer."

"Eventually, they will kneel and come to the negotiation table. We are confident of victory," it added in a statement.

The union blamed Samsung management for "obstructing" the strike, saying they did not appear willing to engage in dialogue.

It urged more workers to participate, including "those who are still hesitant".

"Your determination is needed to advance our goals and victory. Let us unite our strength to protect our rights and create a better future."