WASHINGTON (Reuters) - Despite a steady stream of meetings and calls with U.S. officials, and three revamped proposals to assuage national security concerns, Nippon Steel has failed to garner approval from a powerful panel reviewing its $14.9 billion bid for U.S. Steel, a letter seen by Reuters shows.

The letter, sent Saturday, sets the stage for U.S. President Joe Biden, who has long opposed the deal, to block it. The Committee on Foreign Investment in the United States (CFIUS), which reviews deals for national security risks, has a Dec. 23 deadline to approve the deal, extend the review, or recommend Biden scuttle it.

If the agencies that make up the panel remain at loggerheads, as the letter states, they will refer the matter to Biden to take action.

The history of outreach since early September, including four in-person meetings with CFIUS, three phone calls, including one on Friday with the Treasury and Commerce department secretaries, as well as the three proposed mitigation agreements is contained in a letter dated Saturday sent to Nippon Steel by CFIUS that has not been previously reported.

The White House, Nippon Steel, and the Commerce Department, which is co-leading the review of the deal at CFIUS, did not immediately respond to requests for comment. Treasury, which leads CFIUS, declined to comment. U.S. Steel said in a statement that Nippon Steel provides, "by far, the brightest future for U.S. Steel," adding that no other party can make the billions in investments Nippon Steel has promised to make.

"U.S. Steel will not-- and does not have the resources-- to do this on our own," it added.