The world's largest commercial jetmaker is expected to announce on Monday a $600 million assembly line for its best-selling A320 short-haul aircraft, starting at four planes a month from 2017.

With Airbus already assembling some aircraft in China, analysts expect a more muted labour reaction to the structural move than in 2006, when job cuts on the A380 superjumbo led to street demonstrations and a fracas in the French parliament.

But labour leaders said they would insist that any increase in production to meet rising demand would not come at the expense of output which stands at record levels in Europe.

"We will be vigilant to make sure they are not robbing Europe to pay the United States," said Gilbert Plo, a CFTC union delegate to the company's central works council.

While final assembly is prestigious and visible it only represents some 5 percent of the cost of production, according to Airbus executives. They are likely to argue that each Alabama job will create up to 10 in Europe because that is where the high-value systems and fuselage parts will continue to be built.

Unions will however want those promises written in stone.

"We want guarantees they won't be touching the production rates in Europe — at Hamburg and Toulouse," said Francoise Vallin, a senior official at the CFE-CGC union which represents supervisors and some managers in the Toulouse-based group.

"We also want guarantees that there will be more employment in Europe for the intermediary sections," she added.

Airbus builds large aircraft sections in its four founding nations - Britain, France, Germany and Spain. These are shipped, trucked or flown by Beluga, its giant bulbous-nosed transport plane, to final assmebly lines in Toulouse, France or Hamburg, Germany.

In 2009 Airbus also opened assembly operations in China, extending its production jigsaw by sea to the port of Tianjin near Beijing.

European unions accepted the concept of the Chinese plant as a way of opening doors to the fast-growing Chinese market which is heavily state-controlled. But some will ask questions over the merits of going offshore to speculate on new demand.

An earlier plan to shift some freighters to Alabama was based on hopes of a massive parallel order for U.S. Air Force in-flight refuelling tankers, but Airbus lost that bidding race to Boeing last year.

Under the new project the planemaker is expected to ship sections of the 150-seat A320 to the U.S. Gulf port of Mobile to be assembled by a non-union workforce of up to 1,000 people.

"We would be interested to know who will pay for the increase in logistics costs for this U.S. line," said Vallin.

"Will there be a partnership with the state of Alabama? We are waiting for details to see if it's a good idea or not."

Airbus declined to comment ahead of the expected announcement on Monday.