Boeing moved on Monday to pre-empt any efforts to water down a fierce transatlantic trade clash as its rival Airbus mulls a US$600 million assembly line in the United States.

The world's dominant jetmakers are involved in the largest-ever dispute at the World Trade Organization over mutual accusations of billions of dollars of illegal aircraft subsidies.

Boeing said a U.S. foray being considered by its European rival -- an A320 assembly plant in Mobile, Alabama -- would not dilute the case against European loans that the WTO has found illegal. The same body has also faulted some U.S. aid to Boeing.

"While it is interesting once again to see Airbus promising to move jobs from Europe to the United States, no matter how many are created, the numbers pale in comparison to the thousands of US jobs destroyed by illegal subsidies, which Airbus and its European government underwriters have failed to remove to the satisfaction of the U.S. Government and in direct contravention of international trade law," Boeing's top trade spokesman Charlie Miller said in an emailed statement.

Airbus had initially offered to assemble some commercial freighters in Mobile to sweeten a bid for a bitterly fought US$35 billion refueling tanker contest it lost to Boeing last year.

The latest plan calls for a US$600 million Alabama assembly plant dedicated to the A320 passenger jet that would start at four aircraft a month, one person familiar with the matter said.

It would be the second plant outside Europe for Airbus's most popular jet. The EADS unit produces 37 planes a month between France and Germany and 3 a month in China. By the end of this year it plans to reach 38 in Europe and 4 in China.

EADS Chief Executive Tom Enders said on Thursday no final decision had been taken and called recent reports "speculation".

Sources have not ruled out an announcement as early as Monday ahead of the July 9-15 Farnborough Airshow in the UK, but there are signs some hurdles remain before a deal can be finalized.

"There are plenty of conditions that would need to be met for such a concept to be implemented. This just isn't the case right now," a source familiar with the matter said.