PARIS (Reuters) - French unions called on Wednesday for a strike by Air France-KLM (AIRF.PA: Quote, Profile, Research) staff on May 22 to support national protests planned against proposed pension system reforms, one of France's leading unions said.

Six unions including the two biggest in France, CGT and CFDT, backed the strike and called on staff in Paris and elsewhere to participate in national demonstrations on the pensions issue, the CFDT said in a statement.

It did not say how long the proposed strikes would last.

(Editing by Rory Channing)