American Airlines' shares soared after the firm said it had received an unsolicited offer from Qatar Airways to buy 10 per cent of its stock.
The US airline's Nasdaq-listed shares traded more than five per cent higher in pre-market trading. At the time of publishing they were up 3.35 per cent at $50.05.
Qatar submitted a filing under the Hart-Scott-Rodino Act, which is required for an acquisition of more than approximately $81m (£64m) of American Airline's stock, making it subject to the US Department of Justice's antitrust division.
According to the notice, Qatar Airways intends to purchase at least $808m. In a conversation between the chief executives of the two companies, Qatar indicated that it has an interest in acquiring approximately a 10 per cent stake, American Airlines said.
The US firm will respond in due course, but it said its certificate of incorporation prohibits anyone from acquiring 4.75 per cent or more of the company's outstanding stock without approval from the board.
Qatar Airways said it plans to make an initial investment of up to 4.75 per cent, which it will not exceed without prior consent of the board.
American Airlines also noted that foreign ownership laws limit the total percentage of foreign voting interest to 24.9 per cent.
In an SEC filing, American Airlines said:
"The proposed investment by Qatar Airways was not solicited by American Airlines and would in no way change the company’s board composition, governance, management or strategic direction. It also does not alter American Airlines’ conviction on the need to enforce the Open Skies agreements with the United Arab Emirates and the nation of Qatar and ensure fair competition with Gulf carriers, including Qatar Airways."