WASHINGTON (Reuters) - American companies are scaling back plans to hire workers and a rising share of firms feel the European debt crisis is taking a bite out of their sales, a survey showed on Monday.

Only 23 percent of the firms polled in June plan to add to staff in the next six months, the National Association for Business Economics said on Monday.

NABE's prior survey, conducted in late March and early April, had shown 39 percent of companies planning to add workers.

Already, hiring by U.S. companies has slowed dramatically in recent months as employers worry about a sagging global economy hurt by Europe's snowballing debt crisis.

Some economic data has suggested at least some of the hiring slowdown has been due to caution rather than a decline in business. A July 6 Labor Department report, for example, showed companies asked employees to work longer hours last month, even though they slowed the pace of hiring.

The NABE survey suggests such caution on hiring could continue.

The poll showed 47 percent of companies polled felt their sales have dropped due to Europe's woes.

Among companies that produce goods rather than provide services, the impact was even greater, with nearly four in five reporting a Europe-driven decline in revenues.

Three months earlier, only about a quarter of total firms polled thought sales had fallen.

NABE surveyed 67 of its members between June 14 and June 26. Not all responded to every question. About 40 percent of the firms surveyed have more than 1,000 employees.

(Reporting by Jason Lange; Editing by Chizu Nomiyama)