U.S. manufacturers say the pace of their expansion continued to slow in April, with many factories saying their output is still growing but is crimped by shortages of workers and skills.

The Institute for Supply Management, a trade group of purchasing managers, reported Tuesday that its manufacturing index fell to 57.3 percent last month from March's reading of 59.3. Any score above 50 signals growth.

The categories of new orders, production and employment each fell in April for manufacturers, even though demand for manufactured goods remains robust. Companies surveyed for the index pointed to shortages of raw materials and labor that can cause them to lose out on potential sales.