Domestic cargo traffic for U.S. airlines fell 1.3 percent in September in a new sign that weakness in the American shipping economy was hitting the air transport sector.

The decline in September followed small increases in July and August in the domestic business, according to the Air Transport Association. With a 1.1 percent increase in international cargo traffic, that left cargo for the U.S. carriers flat for the month.

For the full year so far, cargo traffic is up only 0.4 percent.

The persistent weakness in the domestic market has that business down 0.5 percent for the first nine months of this year.

Most of the domestic downturn in air cargo appears to come from the passenger airlines, which have scaled back capacity in U.S. markets this year. American Airlines reported its cargo traffic fell 8.2 percent in September and 6.2 percent in October, although the airline does not break out its domestic and international cargo business.