Prime Minister Shigeru Ishiba has sent a letter to U.S. President Joe Biden asking him to approve Nippon Steel's acquisition of U.S. Steel, to avoid marring recent efforts to strengthen ties between the countries, according to two sources familiar with the matter.

Biden joined a powerful U.S. labor union in opposing the $15 billion takeover of the storied American firm by Japan's top steelmaker and referred it to the Committee on Foreign Investment in the United States (CFIUS), a secretive government panel that reviews foreign investments for national security risks.

The deadline for the CFIUS review is next month, before President-elect Donald Trump — who has vowed to block the deal — takes office on Jan. 20. CFIUS could approve the deal, possibly with measures to address national security concerns, or recommend that the president block it. It could also extend the review.