US regulators are scrutinizing Nippon Steel's proposed takeover of United States Steel. Shareholders of the US firm approved the acquisition at a meeting on Friday.
The Committee on Foreign Investment in the United States is reviewing the takeover bid to determine its impact on the country's national security.
The committee may enter into an agreement with, or impose conditions on, parties to mitigate national security risks or may refer the case to the president for a decision if it decides a transaction presents such risks.
US media report that the Department of Justice has also launched an antitrust investigation of the planned buyout.
Nippon Steel seeks to complete the takeover by the end of September. But necessary procedures may take longer than it expects, depending on regulatory moves.
The United Steelworkers union, or USW, opposes the takeover. US President Joe Biden also expressed his opposition last month.