Kelly Services Inc. executive George Corona started noticing the change about six months ago. The $5.5 billion staffing company was finding it tougher to come up with workers to fill beginner positions at warehouses and call centers run by its clients.

"It's becoming harder and harder to attract people to do these entry-level jobs unless you raise the wages," said Corona, chief operating officer for the Troy, Michigan-based Kelly.

Seven years into the economic expansion, the U.S. is showing some signs of running short of people who want jobs and are qualified to fill existing openings. The shortfall, which has been evident for some time for highly skilled workers such as computer software developers, is starting to spread to those with lesser talents as unemployment falls further.