US workers were less productive in the spring

The drop in productivity helped push unit labor costs up 2.2 percent. That follows a 4.8 percent rise in labor costs in the first three months of this year, the biggest increase since the last three months of 2008.

Rising labor costs reduce corporate profits. Labor represents the largest expense for most companies. And when workers are less productive and cost more, companies are less likely to add jobs.

Economists say a drop in productivity when economic growth has declined is a troubling sign. That likely means companies hired too many workers earlier this year, based on the assumption that growth was picking up. The result: weaker output from a larger work force.

"If demand remains weak, there's a danger that businesses may try to boost productivity by cutting jobs," said Paul Dales, an economist at Capital Economics.