Nippon Steel has postponed its acquisition of United States Steel until December from an initially planned September due to a U.S. review of the deal, the firm said Friday.

The Japanese steelmaker said it and U.S. Steel each received a request for additional information and documentary materials from the U.S. Justice Department in connection with its review of the acquisition.

"Nippon Steel will continue to fully cooperate with the examination of the relevant authorities and are determined to complete the transaction," the company said in a statement.

In December, Nippon Steel announced its plans to acquire U.S. Steel in a deal approved by shareholders of the Pittsburgh, Pennsylvania-based steelmaker in April.

But the transaction drew strong objection from the United Steelworkers union.

U.S. President Joe Biden and his Republican challenger, Donald Trump, have both expressed opposition to the deal as they jostle for votes from steelworkers.

The acquisition is also under scrutiny by the Committee on Foreign Investment in the United States, an interagency committee that investigates national security risks from foreign investments in the U.S.