(Reuters) - The U.S. Department of Justice is seeking more details and documentary materials as part of an antitrust review of Nippon Steel's ( NISTF ) proposed $15 billion takeover of U.S. Steel, the American firm said on Thursday.

Commonly known as a "second request," it can mark the start of a lengthy legal process from which few mergers emerge successful.

The DoJ's request comes after the Japanese firm's proposed bid drew sharp criticism in the U.S., including from President Joe Biden, who said the asset should be domestically owned.

However, an overwhelming majority of U.S. Steel shareholders voted in favor of the deal in April.

Despite fears of layoffs, Japan's largest steelmaker has pledged no job cuts as a result of the merger and has said it will move its own U.S. headquarters to Pittsburgh where U.S. Steel is based.

The acquisition of U.S. Steel will help Nippon, the world's fourth-largest steel maker, move toward 100 million metric tons of global crude steel capacity.

U.S. Steel now expects that the merger will be completed in the second half of 2024, it said on Thursday, as its prior deadline of second to third quarter of 2024 nears.

(Reporting by Ananta Agarwal in Bengaluru; Editing by Anil D'Silva)