One of the front-runners to become Japan's next prime minister has said that any U.S. move to block Japan's Nippon Steel from buying U.S. Steel on national security grounds would be "very unsettling" and could dent trust between the allies.

The White House is close to announcing that President Joe Biden will block the $15 billion deal.

A letter sent to the companies said the takeover could hurt the supply of steel needed for critical U.S. projects and make U.S. Steel less likely to seek tariffs on foreign steel importers.