Hyundai Motor's troublesome unionists are at it again, according to local media reports.

The automaker is well used to a round of summer time strikes coinciding with the annual wage rate negotiations but the latest row - in mid winter - is over a cut in bonus payments, made by the automaker because last summer's strikes caused the 2006 production target to be missed.

More than 1,000 protesting workers rallied earlier on Wednesday Hyundai's headquarters as the government warned of stern measures if the union goes ahead with a threatened strike, Agence France Presse (AFP) reported.

The report said workers, waving banners, wearing red headbands and shaking fists, chanted slogans urging the payment of the disputed bonus as about 100 company security guards formed a human wall at the entrance to the office building and some 1,500 riot police stood guard. Dozens of police buses were parked tightly around the compound to keep the workers at bay, AFP added.

The news agency said the workers mostly had been bussed in from Hyundai's main plant at Ulsan 400 kilometres (250 miles) southeast of Seoul and three representatives were later admitted to deliver a letter of protest to the automaker's executives.

AFP said the latest clash between managers and the 44,000-strong union began when the automaker cut year-end bonus payments by a third because the production target was missed last year. The union called an overtime ban in protest and has given the company until Thursday to pay the full bonus or face a strike vote.

Labour Minister Lee Sang-Soo told the news agency the government would crack down hard should the union carry out its threat to walk out.

"We hope the union and management settle the issue through dialogue at the earliest possible date," Lee told AFP, adding: "But if the union resorts to violence or stages an illegal strike, the government will strongly deal with it."

According to the report, each side has sued the other over the dispute, with management claiming $US1m compensation for lost production.

AFP noted that nine previous company lawsuits against the union since 2000 were dropped following union pressure but the firm has vowed not to back down on this occasion.

Since the militant union was launched in 1987, strikes have cost the company some $10bn dollars in losses, AFP said, citing the Yonhap news agency.

Hyundai Motor said it had been losing daily production of more than 1,000 vehicles worth some 15bn won ($16m) since the latest overtime ban began on 28 December, Agence France Presse added.