A union representing tens of thousands of workers at Samsung Electronics in South Korea on Wednesday said 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. (AFP)