Qatar Airways is attempting to buy a 10 percent stake in American Airlines, which has confirmed the move by filing for a required review by the US justice department's antitrust division.
American Airlines said in the regulatory filing on Thursday that the bid was unsolicited, but that the CEOs of both airlines have spoken.
"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," said American Airlines.
Qatar plans to buy at least $808m in common stock of American Airlines. American Airlines' shares are listed on the Nasdaq and the airline said that Qatar Airways plans to make its purchases on the open market.
The US limits foreign ownership of airlines to less than 25 percent.
American Airlines and Qatar Airways are already members of the Oneworld marketing alliance, which allows passengers to earn and redeem points on each other's flights.
An expanded partnership could make it easier for American Airlines passengers to get to smaller cities in India, and developing countries in Southeast Asia.
Shares of American Airlines shot up 5.2 percent in pre-market trading to $50.90.
American Airlines' certificate of incorporation restricts anyone from buying 4.75 percent or more of its shares without advance board approval following a written request. American Airlines says it has not received such a request yet.
Qatar Airways said later on Thursday that the initial investment would be limited to 4.75 percent.
"Qatar Airways believes in American Airlines' fundamentals and intends to build a passive position in the company with no involvement in management, operations or governance," it said.
Qatar Airways last year set up a revenue-sharing partnership with British Airways parent International Airlines Group, deepening its partnership with that company. It owns just over 20 percent of IAG, which also controls European carriers Aer Lingus, Iberia and Vueling.
In July, Qatar Airways announced a deal to buy a 49 percent stake in Meridiana, Italy's second-biggest carrier. And in December it announced a 10 percent stake in Chile's LATAM Airlines Group, acquired for $608m.
Source: News agencies