By Jeff Mason

WASHINGTON (Reuters) -U.S. President Joe Biden is preparing to block Nippon Steel's acquisition of U.S. Steel, a source said on Wednesday.

The move by Biden would deal a blow to the proposed $14.9 billion deal.

The president has previously expressed concern over Nippon's deal to buy the 122-year-old iconic U.S. steelmaker.

U.S. Steel warned earlier on Wednesday that a failure to merge with Nippon Steel would put thousands of U.S. union jobs at risk.

(Reporting by Jeff Mason; writing by Jasper Ward; editing by Rami Ayyub)