TOKYO (Reuters) - Japanese companies, especially those considering investing in the U.S., are closely watching Washington's review of Nippon Steel's proposed $15 billion acquisition of U.S. Steel, Japan's biggest business lobby Keidanren said on Friday.

"We sincerely hope that the review procedure (in the United States) will be conducted with the utmost integrity and fairness in accordance with the law," Keidanren said in a statement.

The deal, facing growing bipartisan political opposition ahead of the U.S. presidential election, has caught the attention of Japanese companies as Washington is Tokyo's closest ally and Japan is the biggest foreign investor into the U.S.

Sources told Reuters on Wednesday the White House was close to announcing that President Joe Biden will block the Japanese company's bid for U.S. Steel on national security risks.

Keidanren's statement comes after the Japan-U.S. Business Council, a Washington, D.C.-based industry group representing major Japanese firms, said on Thursday it was seriously concerned by the reports Biden was going to block the sale.

(Reporting by Katya Golubkova; Editing by Himani Sarkar and Sonali Paul)