South Korea's Hyundai Kia Automotive Group was hit by fresh strikes on Thursday as thousands of workers downed tools to push for wage reforms, with reports that the action would cost it more than $100 million.

Around 13,000 staff at Hyundai Motor's Ulsan plant worked just a quarter of their eight-hour morning shift, while those on the late rota are expected to follow suit.

The action comes after union members at Hyundai and its subsidiary Kia Kia Motors Corp. walked out on Friday, working just six of their eight hours.

Unions representing the two firms have also called for a massive rally outside Hyundai Kia Automotive Group's headquarters in Seoul, a union spokesman told AFP.

The walk-out comes after 17 rounds of talks with the management over the past two months have produced little progress.

Unions are calling for the firms to count regular bonuses as part of ordinary wages. With ordinary wages used as the basis for setting allowances such as overtime and holiday shift pay as well as paid annual leave and pensions, a deal would boost incomes by an average 10 percent.

But the company insists the bonuses, which are paid to workers only if they meet performance targets, cannot be seen as part of ordinary, fixed wages.

The walkout on Thursday alone is expected to cost Hyundai Motor some 5,400 cars in lost production and 110 billion won ($108 million) in sales, a Hyundai Motor spokeswoman said.

The Hyundai Motor union voted for the strike on August 14, with 70 percent of 47,262 union members voting in favour of industrial action.

Kia said Friday's action led to the loss of production of 1,300 cars worth 22 billion.