Samsung Electronics' labor union said Tuesday it will launch a four-day strike from Liberation Day this week, stepping up pressure over their demand for a pay raise and additional bonuses.

The National Samsung Electronics Union, the largest labor union of the South Korean tech giant, announced its plan to stage the strike on Aug. 15-18, instructing its striking members to refuse to work on Liberation Day on Thursday and to work in shifts until Sunday.

The union and management have held several rounds of talks since January, but have been unable to narrow their differences over the wage increase rate, vacation system and bonuses.

The NSEU, representing 31,000 workers, or about 24 percent of Samsung Electronics' 125,000-strong workforce, has been on a full-scale strike since July 8.

Earlier this month, the unionized workers returned to work after a 25-day strike and vowed to shift to a "long-term plan" that includes staging "blitz" walkouts.

The union is demanding a 5.6 percent basic pay raise for all members, a guaranteed day off on the union's founding day and compensation for economic losses due to the strike.

Samsung Electronics has offered a 5.1 percent increase in wages while emphasizing its commitment to building a win-win relationship with the labor union. (Yonhap)