(Reuters) - American Airlines’ chief executive said on Thursday he is not “particularly excited” about Qatar Airways’ interest in buying up to 10 percent of the U.S. carrier’s shares, in a letter to employees following the state-owned Gulf airline’s overture.

The move by Qatar Airways would expand its investments in North America as Qatar is embroiled in the region’s worst diplomatic crisis in years and is locked in an airspace rights row with three other Gulf states.

Separately, American Airlines Group Inc AAL.O is already part of a push by U.S. carriers to squeeze Qatar out of their domestic market.

Along with United Continental Holdings Inc UAL.N and Delta Air Lines DAL.N, American has pressed the U.S. government to curb U.S. flights by Qatar Airways and rival Gulf carriers Emirates [EMIRA.UL] and Etihad Airways.

The U.S. carriers charge that their Gulf rivals have received billions of dollars in unfair state subsidies, allegations the Gulf carriers deny.

In his letter, American CEO Doug Parker promised to continue American’s “full court press ... to stand up to companies that are illegally subsidized by their governments.”

He also said he found Qatar Airways’ proposed investment “puzzling given our extremely public stance on the illegal subsidies that Qatar, Emirates and Etihad have all received over the years from their governments.”

Qatar Airways responded on Twitter, saying: “We are glad to see American Airlines’ CEO Doug Parker’s perspective that he agrees with Qatar Airways’ belief that American Airlines is a solid financial investment.”

The potential investment is worth at least $808 million, American said in a regulatory filing on Thursday, and would put Qatar Airways' stake on par with Warren Buffett's Berkshire Hathaway Inc BRKa.N, which holds a 10 percent stake in the airline.

FILE PHOTO - American Airlines aircraft are parked at Ronald Reagan Washington National Airport in Washington, U.S., August 8, 2016. REUTERS/Joshua Roberts/File Photo

Shares of American Airlines rose more than 5 percent in pre-market trade after it disclosed the potential investment. The stock closed up 1.1 percent.

Qatar Airways said in a statement that it sees a “strong investment opportunity” in American and that it “intends to build a passive position in the company with no involvement in management, operations or governance.”

“Qatar Airways plans to make an initial investment of up to 4.75 percent. Qatar Airways will not exceed 4.75 percent without prior consent of the American Airlines board. Qatar Airways will make all necessary regulatory filings at the appropriate time,” it said.

American, in its filing, noted potential obstacles to Qatar’s plan, as its rules prohibit “anyone from acquiring 4.75 percent or more of the company’s outstanding stock without advance approval from the board.” It said it had received no request from Qatar for such approval. Further, American said, “there are foreign ownership laws that limit the total percentage of foreign voting interest to 24.9 percent.”

AMERICAN SHARES CHEAPER THAN RIVALS

A stake in American Airlines would add to Qatar Airways' investment portfolio. The Middle East's second biggest airline also owns 20 percent of British Airways-owner International Airlines Group ICAG.L and 10 percent of South America's LATAM.

American Airlines shares are cheaper than Delta, United and JetBlue JBLU.O on a forward 12-month earnings per share basis, according to Thomson Reuters data, and are the cheapest among the top eight U.S. airlines.

Qatar Airways Chief Executive Akbar al-Baker has said the investments were purely financial, though he has looked for opportunities to cut costs or expand service with the oneworld alliance airlines in which it owns a stake.

Qatar Airways, American Airlines, IAG’s British Airways, Iberia and LATAM are all members of the oneworld airline alliance.

British Airways and Qatar Airways have a revenue-sharing partnership between their respective hubs in Doha and London, and Qatar Airways plans to launch flights to LATAM’s base in Santiago, Chile.

“The U.S. market is strategically important to Qatar Airways and this would strengthen their ability to feed at the U.S. end,” independent aviation consultant John Strickland told Reuters. “However, if it does go ahead it would not give them automatic antitrust immunity. That would have to be negotiated separately.”

Qatar Airways’ desire to invest in American could be as much a political decision as financial, analysts said, as Saudi Arabia, the United Arab Emirates, Bahrain and Egypt have closed their airspace to Qatar Airways, forcing it to cut flights to those countries and fly longer, more expensive, routes.

Those countries cut diplomatic and transport ties with Qatar earlier this month, accusing it of fomenting regional unrest, supporting terrorism and getting too close to Iran, all of which Doha denies.

“There’s a good chance this investment plan by Qatar Airways may have more to do with demonstrating sovereign Qatar’s continued commitment to the United States amid the latest Gulf States feud with Qatar,” Gimme Credit analyst Vicki Bryan wrote in a research note.

Al-Baker, highly critical of the blocking of airspace, has said Qatar Airways would use the aircraft used to fly to those countries for fast-track expansion plans elsewhere.