By Nick Carey

DETROIT (Reuters) - Higher U.S. tariffs on imported steel will mean more jobs at some American steel mills but raise the risk of layoffs at others, industry executives said on Wednesday.

On the winning side of President Donald Trump's plans to impose a 25 percent tariff on imported steel are U.S. steel mills with excess capacity that can be restarted, albeit with a significant delay, to meet increased demand. On the losing side are steel makers that rely on the import of specialty steel products not manufactured in the United States.

Bob Miller, chief executive officer of the U.S. operations of Novolipetsk Steel PAO, knows exactly how his business and workers will be affected - and it is not good news.

"This will absolutely lead to layoffs," Miller said, explaining that U.S. producers do not make the steel slabs he needs to supply sheet steel to customers like Caterpillar Inc and automakers.

If Trump's steel tariffs are imposed without exemptions, Miller says Russia-based Novolipetsk Steel will shelve plans to invest $600 million to add capacity at factories in Pennsylvania and Indiana and lay off hundreds of American steel workers. Within three to four months the company would have to cut 50 percent of the 1,200 steel workers it employs today because it would run out of raw materials, he said, adding that if customers refuse to accept higher prices, the company's U.S. "business case could disappear."

"Personally, I think my 1,200 U.S. steel jobs are just as important as anyone else's 1,200 steel jobs," Miller said.

But at U.S. Steel Corp the company is readying to ramp up operations in expectations that the tariffs will spur demand. U.S. Steel on Wednesday said it would begin restarting one of two idled blast furnaces at its Granite City, Illinois, steel plant – a process that will take up to four months and could support 500 jobs.

The company is "actively reviewing the anticipated increased demand for steel produced in the United States," a U.S. Steel spokeswoman said.

The two companies are emblematic of how the proposed tariff on steel has exposed division within the industry and highlighted the challenges of rebuilding capacity in a capital-intensive sector.

U.S. steel makers have said the domestic industry has 30 million tons of annual idled capacity and can ramp up to meet demand - which totaled around 107 million tons in 2017, 36 million of which was imported.

Granite City and an AK Steel Holding Corp mill in Ashland, Kentucky, are the only two idle U.S. plants sufficiently well maintained to be brought back any time soon, industry analysts said. Between them, they have 5.3 million tons of capacity available.