BRUSSELS - Europe's airlines appear to have just suffered their worst month yet of the downturn, with preliminary traffic figures down 9 percent year-on-year in May, the Association of European Airlines said on Monday.

AEA's weekly figures for the last four weeks of May and the first week of June are relentlessly poor, with May likely to produce a decrease in the order of 9 percent, said the group, which represents 34 major carriers.

The statement comes after the head of European airports body ACI Europe told Reuters in an interview last week it was slashing its 2009 traffic forecast to an 8 percent decrease for passengers and a 16 percent decrease for freight.

But Boeing's (BA.N) chief executive Jim McNerney told European newspapers ahead of the Paris Airshow that the economic crisis was unlikely to worsen.

AEA Secretary General Ulrich Schulte-Strathaus said all three of its members' markets -- intra-Europe, the North Atlantic and the Far East -- were suffering.

A positive recent development has been an increase in the tempo of capacity reduction. In April, seat-kilometres offered was 2.8 percent down on last year, while the preliminary May figure is down 5.3 percent, though still below the traffic decrease.

The four months reported so far are pieces of a jigsaw which, put together, reveal a 6 percent drop in traffic compared to the first four months of last year, Schulte-Strathaus said.

The next piece in the jigsaw will reveal an even worse scenario, he added. (Reporting by Pete Harrison, editing by Will Waterman)