WASHINGTON — American factories grew at a slightly faster pace in May, another sign that manufacturing remains healthy.

The Institute for Supply Management says its manufacturing index rose to 54.9 last month from 54.8 in April. Anything above 50 signals that factories are expanding. They have now registered nine straight months of growth. May’s improvement was unexpected: Economists had expected the index to fall slightly last month.

New orders and hiring grew faster in May. Production and export orders grew more slowly.

Fifteen of 18 manufacturing industries posted growth last month, led by minerals and furniture companies.

U.S. factories have largely recovered from a slump in late 2015 and early 2016 caused by cutbacks in the energy industry and a strong dollar, which makes U.S. goods more expensive in foreign markets.