About 55,000 workers at South Korea's largest car manufacturer, Hyundai Motor. and its affiliate Kia Motors, stopped work for two hours, union officials said.

A Hyundai spokesman said the strike was expected to cost the companies about $40.4m in lost production, or 2,900 vehicles.

"This is not a political strike, but a strike that is aimed at protecting our right to health," Lee Suk-haeng, the leader of the Korean Confederation of Trade Unions which organised the strike, told reporters.

Protesters warned

The union leader said his group planned to launch a nationwide consumer campaign to boycott US beef.

About 55,000 workers stopped work for two hours [AFP]
The president's office denounced the walkout as an attempt to hurt the nation's economy and vowed to deal sternly with strikes.

"Now it is time for everyone to return to work and pull together all our energies and wisdom to surmount economic difficulties," the office said in a statement.

US beef sales resumed on Tuesday, although they were limited to one store run by a US beef importer.

Lee agreed to lift the import ban in April imposed in 2003, when the first case of mad cow disease was discovered in the US.

This led to a backlash due to health concerns and a perception that South Korea had backed down too easily to US pressure.

As the protests peaked in June at 80,000 people, the South Korean cabinet offered to resign and Lee reshuffled top aides.