Employers say they're having a tougher time filling positions for engineers, machinists and other specialized workers, according to Manpower, a Milwaukee, Wis.-based employment services firm.

In a survey of 2,000 employers nationwide, engineers were cited as the most difficult jobs to fill, followed by machinists, skilled trades workers, technicians and sales representatives. Other jobs, including IT workers and production operators, appeared on the list for the first time, the company said.

The U.S results were part of a broader survey of 42,500 employers in 32 countries around the world. At the global level, skilled trades, sales reps and technicians were the hardest jobs to fill.

Manpower Vice President Melanie Holmes said that as baby boomers retire, employers need to focus their recruiting efforts on attracting younger workers.

"It is essential for companies to find a balance where they are attracting and retaining aging workers, while still developing innovative recruiting programs targeting young professionals, especially those interested in technical and trade careers," she said in a statement.