Some non-unionised truckers also took part in the walkout, the union and the government said.
The strike has already caused $11 million in losses to exporters and $3 million to importers, according to the Korea International Trade Association, a private association of local exporters and importers.
A two-week strike by about 6,000 transporters in May 2003 caused $540 million in losses to exporters, the association said.
Port operations hit

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Up to 24,690 containers were moved in or out of the country's largest sea port of Busan on Friday, compared with the daily average of 34,290 before the strike, according to the ministry of land, transport and maritime affairs.
Busan handles more than 70 per cent of South Korea's container traffic.
Daily container traffic dropped by about 90 per cent in the western port of Pyeongtaek and the southern port of Gwangyang, Hong Seok-ku, a ministry official, said.
Local television footage showed the docks clogged with containers piled high upon each other.
Earlier this week, union leaders and government officials held several rounds of negotiations on the transporters' demands but failed to reach a breakthrough.
The government dispatched military vehicles and used trains to transport cargo, and also plans to send police if striking workers prevent cargo from entering sea ports and engage in other illegal activities.