The US invoked Section 232, a rarely used tool that allows emergency trade sanctions on "national security" grounds. US President Donald Trump in April ordered an investigation into whether foreign steel imports affect national security.

"The government told steelmakers that they need to set up business plans by taking into account Washington's stronger trade protectionism," a ministry official said.

Industry watchers estimate that exports of South Korean steel are likely to reach 3.5 million tons this year, down from 3.74 million tons tallied for last year.

The volume of steel exports to the US fell, hit by Washington's imposition of tariffs, but the value of shipments rose as steel prices hiked.

In particular, export volumes and the prices of South Korean steelmakers' oil country tubular goods increased, aided by a boom in the US oil and gas industry.

The US probe into foreign steel imports is likely to come out early next year.

South Korea's trade ministry plans to consider filing complaints with the World Trade Organization if the US takes "unfair" measures to limit imports of South Korean steel products after the investigation. (Yonhap)