The European Union's plan to force international airlines to buy permits for their carbon dioxide emissions is in line with international law, Europe's highest court said in a preliminary opinion Thursday.

Several U.S. and Canada-based airlines and airlines associations had sued the EU for its plan to include them in the emissions trading scheme as of next year. Under that plan, all airlines would have to have emissions permits for flights to and from EU airports. So far only large factories and power plants are part of the scheme.

Among several complaints, the airlines had argued that the EU did not have the jurisdiction to require permits for emissions produced during the entire flight, or even stretches that cross the air space of non-EU countries.

But the ECJ's Advocate General Juliane Kokott disagreed with their arguments Thursday.

"The inclusion of international aviation in the EU emissions trading scheme is compatible with the provisions and principles of international law invoked," she said in her opinion to the court. The advocate general's opinion is not binding for the court but is followed in most cases. A final ruling usually comes several months after the opinion.

The emissions trading scheme is the EU's main tool in its fight against climate change. The EU has argued that it planned to include all airlines into its scheme since most other major states and regions do not have a similar system. Forcing only European airlines to buy permits would have put them at a competitive disadvantage compared to their foreign competitors and undermined the effectiveness of the scheme.

"I am glad to see that the Advocate General's opinion concludes that (the) EU Directive is fully compatible with international law," the EU's Commissioner for Climate Action Connie Hedegaard said. "The EU reaffirms its wish to engage constructively with third countries during the implementation of this legislation."

The Air Transport Association of America industry group rejected Thursday's legal opinion.

"ATA's view that the extension of this unilateral, regional scheme to aviation violates international law is supported by more than 20 countries," including Brazil, Russia, India, China and Japan, the ATA said in a statement.

It said Thursday's development was "a non-binding, preliminary opinion, it does not mark the end of this case."