A key index of U.S. manufacturing fell in April more than expected, the Institute for Supply Management said Wednesday.

The ISM's purchasing managers index fell to 50.7 from March's 51.3, more that the 50.9 expected by analysts polled by Thomson/Reuters I/B/E/S.

The 0.6 percentage point decline in the growth of U.S. manufacturing puts the sector's expansion at its lowest level so far this year, the ISM said.

"The slowdown in global economic growth appears to have hit American manufacturers in April," Ama Asaf, economist with London-based Capital Economics, said.