Turkey on Wednesday signed a letter of intent opening the way to Ankara buying ground to air missiles from a French-Italian consortium.

The letter signed by the Turkish, French and Italian defence ministers allows the Eurosam consortium to "analyse and define the needs" of the Turkish military, a French source told AFP.

Though the deal is at an extremely early stage, NATO Secretary General Jens Stoltenberg welcomed it as positive cooperation between alliance members, after Ankara caused alarm by agreeing in September to buy Russian S-400 missile systems.

"We welcome always when NATO allies are working together to develop different capabilities," Stoltenberg said.

"This kind of operation is the best way to make sure we have the capabilities different nations need and it's a good way also to make sure that when we have new capabilities they can be fully integrated into air defence systems."

The letter signed on the sidelines of a NATO meeting in Brussels allows the three countries to "show their interest in cooperating in the domain of anti-aircraft defence and surface-to-air missiles", the French source said.

Eurosam air defence systems are already used by the French and Italian militaries and Rome even deployed some for a time in Turkey to help intercept missiles fired from Syria.