By Kathleen Madigan 

Two surveys of businesses outside of manufacturing indicate output growth and employment strengthened at the start of the second quarter, suggesting the U.S. economy is revving up after possibly contracting in the first quarter.

The Institute for Supply Management said Tuesday its non-manufacturing purchasing managers index came in at 57.8 in April, up from 56.5 in March and better than the 56.3 expected by forecasters surveyed by The Wall Street Journal.

Earlier Tuesday, data provider Markit said its service-sector composite slowed to 57.4 in April from 59.2 in March, but the April index stood above the first-quarter average of 56.8. Markit said its employment index jumped to a 10-month high. As with the ISM, Markit readings above 50 indicate activity is expanding.

In the ISM report, "the majority of respondents indicate that there has been an uptick in business activity due to the improved economic climate and prevailing stability in business conditions."

"The index's trend has been a consistent pattern of slow incremental growth," said Anthony Nieves, who oversees the ISM survey.

The ISM's new orders index increased to 59.2 in April from 57.8 in March. The exports index, however, plunged to 48.5 after it jumped to 59.0 in March. Mr. Nieves indicated that only a small portion of non-manufacturers sell overseas, but said the strength of the dollar has impacted exports.

Production reversed course sharply after two months of slowing. The ISM business activity/production index rose to 61.6 from 57.5 in March.

A sharp widening in the March trade deficit reported earlier Tuesday caused many economists to estimate the U.S. economy shrank in the first quarter. The ISM report suggests better growth this quarter.

"Today's report is an encouraging sign that, after a distorted first quarter, the economy is getting back to a solid uptrend," wrote Stephen Stanley, chief economist at Amherst Pierpont Securities, in a research note.

Like the Markit survey, the ISM report was upbeat about payrolls ahead of Friday's April employment report. The ISM employment index edged up to 56.7 in April from 56.6 in March. That is in contrast to last Friday's ISM manufacturing survey, which found a contraction in factory payrolls.

Non-manufacturers continue to think their inventories are too high, but Mr. Nieves said the rising backlog of orders and strong production readings suggest the buildup in inventories is intentional, as businesses are busy meeting new demand.

The prices index slowed to 50.1 from 52.4.

The ISM non-manufacturing report is comprised mainly of comments from service-sector companies that make up the bulk of the U.S. economy, but it also includes construction and public administration.

Write to Kathleen Madigan at kathleen.madigan@wsj.com