U.S. manufacturing activity unexpectedly picked up in August amid strong order growth, but a measure of factory employment dropped to a nine-month low, likely as workers remained scarce.

The survey from the Institute for Supply Management (ISM) on Wednesday continued to highlight persistent problems securing enough raw materials, a situation worsened by disruptions caused by the latest wave of COVID-19 infections, primarily in Southeast Asia, as well as ports congestion in China.

"A surprising turn of events for manufacturing activity in the U.S., but it doesn't change the story of supply disruptions and shortages holding back stronger growth," said Jennifer Lee, a senior economist at BMO Capital Markets in Toronto.

The ISM said its index of national factory activity inched up to 59.9 last month from a reading of 59.5 in July. A reading above 50 indicates expansion in manufacturing, which accounts for 11.9 percent of the U.S. economy. Economists polled by Reuters had forecast the index falling to 58.6.

Manufacturing is holding up even as spending is rotating back to services from goods because of vaccinations against COVID-19. All of the six largest manufacturing industries, including computer and electronic products, chemical products and transportation equipment, reported moderate to strong growth.

Manufacturers of computer and electronic products said while a global semiconductor shortage was impacting supply lines, they had so far "been able to manage it without impacting clients."

Chemical goods producers said they continued to "see extended lead times due to port delays and sea container tightness." Transportation equipment makers reported that "strong sales continue, but production is limited due to supply issues with chips."

The ISM survey's forward-looking new orders sub-index rebounded to a reading of 66.7 last month after two straight monthly declines. Fourteen out of 18 manufacturing industries, furniture and related products, machinery and electrical equipment, appliances and components reported growth in new orders. Only nonmetallic mineral products reported a drop.