U.S. Steel would close mills and likely move its headquarters out of Pittsburgh if the $14.9 billion buyout by Nippon Steel collapses, the Wall Street Journal reported on Wednesday citing an interview with the company's CEO.

The steelmaker's CEO David Burritt told WSJ the nearly $3 billion Nippon had pledged to invest in U.S. Steel's older mills was crucial to remain competitive and maintain workers' jobs.

"We wouldn't do that if the deal falls through," Burritt told the WSJ. "I don't have the money."

The deal has come under increasing criticism from U.S. politicians and the United Steelworkers union since it was announced in December.

On Tuesday, Democratic presidential candidate Kamala Harris said U.S. Steel should remain in domestic hands, while Republican nominee Donald Trump said he would move to block the deal if elected.

U.S. Steel and Nippon Steel have tried to assuage concerns around the deal and have touted its benefits.

Earlier on Wednesday, Nippon said the core senior management as well as a majority of board members at the U.S. company would be U.S. citizens, if the deal goes ahead.

Burritt told WSJ the expanded Arkansas mill would allow the company to close Mon Valley, its last steelmaking operation in Pittsburgh. The company would likely look to move its headquarters to the South too.

The deal has received all regulatory approvals from outside the U.S. and a greenlight from U.S. Steel's shareholders. It is now under the regulatory review process in the United States.