"The head of Qatar Airways sunk to a new low recently when he disparaged hard-working us crewmembers and airlines that represent $1.5 trillion in economic activity", ALPA President Tim Canoll said in a statement.
The decision "is an extension of our stance against illegal subsidies these carriers receive from their governments", it said. The Middle Eastern airline called American's choice anti-competitive and anti-consumer, as it "may result in higher fares for travelers to and from the U.S".
Etihad said it was "disappointed" with American Airline's exit from the codeshare pact and rejected allegations it violated any air-transportation agreements.
A Qatar Airways A350 press conference with Qatar CEO Akbar Al Baker at center.
The decision comes as Qatar Airways confirmed it would go ahead with plans to buy a stake in American Airlines.
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American says it didn't solicit the investment.
Qatar Airways has announced that it will launch a new direct service to Kiev, the capital of Ukraine, from August 28, 2017. On top of its impending equity stake in America, Qatar Airways owns 20% of IAG - the parent company of British Airways and one of American's closest partners.
Akbar Al Baker, the chief executive of Qatar Airlines, has apologized for calling US flight attendants "grandmothers".
Al-Baker previously said Qatar Airways would buy American shares on the open market before formally seeking board approval from the USA carrier to increase its ownership.
"Etihad Airways and numerous leading consumer and business groups in the United States have categorically rejected the allegations of American and the two other dominant USA carriers regarding violations of the relevant air transportation agreements".
Qatar Airways, like American, is a oneworld global alliance member so interlining, frequent flyer and lounge benefits remain pertinent.