Workers at three GM plants in South Korea have walked out on strike seeking higher wages and performance-based bonuses.

Unions have asked for an increase of KRW150,611 (US$140) a month in basic salary and 30% of last year's net profit in profit sharing.

GM Korea has offered a rise of KRW68,828 in basic wages and KRW2.2m per employee in one-time bonuses.  Company spokesman Kim Sang-won told Dow Jones Newswires: “"We will continue negotiations with the union to strike a wage deal before the summer holidays, which fall in the first week of August."

GM Korea workers at Bupyeong, Gunsan and Changwon plants began a six-hour strike on Tuesday and are planning a three-hour strike at the three plants on Wednesday, according to GM Korea's union, which has 10,000 members.

The company estimates production losses equivalent to 3,600 vehicles from the two stoppages.

The car maker's management and union have held 12 rounds of negotiations since 25 May.

GM Korea said it has not been able to set a date for the next round of talks with union leaders, but added that every effort will be made to resolve outstanding differences as soon as possible.