Involuntary part-time unemployment soared in May, the Bureau of Labor Statistics reported Friday, in one of the more discouraging datapoints from the monthly jobs report.

The number of people working part-time because their hours were cut or because they couldn't find a full-time job rose by 468,000 to 6.4 million in May, according to the report.

The number of people forced into part-time work was only 119,000 lower than a year before, and up 611,000 from September of last year, when it hit a post-recession low.

Workers who want more hours are considered to be "underemployed," and the number of involuntary part-time workers is viewed by officials at the Federal Reserve and elsewhere as an indication of "slack" in the labor market, or evidence that workers are still hurting from the recession and that the recovery is not yet complete.

The data, taken from the household survey, can be volatile from month to month, and a one-month jump in involuntary part-time work is not unheard of.

Nevertheless, it was one of several disappointing indicators from the May report.

While the headline unemployment rate fell from 5 percent to 4.7 percent, that improvement was driven in part by shrinking labor force participation, which decreases the denominator in the calculation of unemployment.

A broader underemployment rate, the U-6 unemployment rate, showed underemployment steady at 9.7 percent. The U-6 rate takes into account not just workers who can't find jobs, but also people forced into involuntary part-time work or who search for jobs only sporadically.

The underemployment rate has fallen from as high as 17.1 percent during the worst of the recession to 9.7 percent today. But progress has mostly stalled out since October, when it hit 9.8 percent.