TOKYO (Reuters) - Japanese Prime Minister Shigeru Ishiba has sent a letter to President Joe Biden asking him to approve Nippon Steel's ( NISTF ) 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. labour union in opposing the $15 billion deal 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.

"Japan stands as the largest investor in the U.S., with its investments showing a steady upward trend. Continuing this upward trend of Japanese investment in the U.S. benefits both of our countries, showcasing the robustness of the Japan-U.S. Alliance to the world," Ishiba said in the letter, according to a copy of the text seen by Reuters. The sources confirmed it was sent to Biden on Nov. 20.

"Under your presidency, this Alliance has reached unprecedented strength. We respectfully ask for the U.S. government to approve the planned acquisition by Nippon Steel ( NISTF ) so as not to cast a shadow on the achievements you have accumulated over the past four years," the letter said.

The U.S. embassy in Japan declined to comment. Ishiba's office deferred questions to the foreign ministry which did not immediately have comment. Nippon Steel ( NISTF ) declined to comment and U.S. Steel did not immediately reply to a request for comment outside of U.S. business hours.