The Markit Economics preliminary index of U.S. manufacturing increased to 56.2 in May from 55.4 a month earlier as output accelerated, the London-based group said.

Readings above 50 for the purchasing managers’ measure indicate expansion and the May figure was the highest in three months. The median forecast in a Bloomberg survey of 21 economists was 55.5, with estimates ranging from 54.1 to 56.5.

“The U.S. manufacturing sector continued to gain strength heading into mid-year as supportive demand conditions led to the sharpest month-on-month increase in production for over three years,” Paul Smith, senior economist at Markit, said in a statement. The “industry will aid a rebound in U.S. GDP in the second quarter.”

The group’s gauge of orders declined to 58.2 in May from 58.9. A measure of factory employment decreased for a third month to the lowest level since January.

The Markit measure is based on replies from about 85 percent to 90 percent of companies in a poll of more than 600 American manufacturers.