Smoke billows from chimneys as factories churn in South Korea's steelmaking heartland, now under threat from Washington's swingeing 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 fuelled 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 percent of its total steel imports.

But the industry has faced intense strain in recent years from foreign competition.

And businesses, officials and workers in the city now fear a planned 25 percent tariff on all steel imports to the United States beginning next month could have devastating impacts -- and major knock-on effects on South Korea's economy.

"The steel industry is a vital national industry that serves as a fundamental material for key sectors such as construction, automotive and shipbuilding," Pohang's mayor Lee Kang-deok told AFP.

"If the steel industry collapses, the entire South Korean economy will be destabilised," Lee warned.

"If........

© International Business Times