(Reuters) -U.S. President Joe Biden plans to formally block Nippon Steel's proposed takeover of U.S. Steel on national security grounds once the $15 billion deal is referred back to him later this month, Bloomberg News reported on Tuesday, citing people familiar with the matter.

Asked about Biden's plans on the deal, which is being reviewed by a national security panel, White House spokesperson Karine Jean-Pierre declined to comment.

The Committee on Foreign Investment in the United States (CFIUS) panel reviewing the deal must refer its decision on the merger to Biden by Dec. 22 or 23, Bloomberg added.

In March, Biden said it was "vital" for U.S. Steel to remain American-owned. President-elect Donald Trump said last week that he would block the deal when he takes office next month.

Despite opposition, including from the United Steelworkers Union, Nippon has pressed on, promising to not transfer any U.S. Steel production capacity or jobs outside the U.S. if the merger succeeds.

Nippon has also said it would not interfere in any of U.S. Steel's decisions on trade matters, including decisions to pursue trade measures under U.S. law against unfair trade practices.