Smoke billows from chimneys as factories churn in South Korea's steelmaking heartland, now under threat from Washington's harsh new tariffs on the port city's largest export.

The city of Pohang on South Korea's east coast for decades pumped out the steel that fueled the country's breakneck economic rise.

South Korea was the fourth largest exporter of the metal to the United States last year, accounting for 13% of its total steel imports.