Joblessness in King and Snohomish counties rose in March from a year ago even as more people found work, data released Tuesday by the state Employment Security Department show.

In both counties, an increase in total employment was offset by a surge in the pool of available workers amid signs of a strengthening economy.

The non-seasonally-adjusted unemployment rate in King inched up to 5.2 percent in March from 5.1 percent a year ago, though it was unchanged from February. A year-over-year increase of 2.8 percent in the county’s labor force, which includes people who are working or actively looking for work, was nearly matched by a 2.6 percent rise in total employment, the data show.

Joblessness in Snohomish County was unchanged over the month at 6.0 percent, but climbed from 5.7 percent in March 2013. The county’s labor force grew 2.9 percent over the year, while total employment expanded 2.6 percent.

King and Snohomish were the only two counties statewide with year-over-year increases in unemployment last month, according to Employment Security.

Experts say a strengthening regional economy is pulling discouraged job seekers back into the labor market, nudging unemployment rates upward.

The county unemployment data are not seasonally adjusted for month-to-month fluctuations and are based on a household survey.

As reported last week, the state’s seasonally adjusted unemployment rate was 6.3 percent in March, the same as in February. On a non-seasonally-adjusted basis, the statewide rate was 6.9 percent in March, down from 7.3 percent a month earlier and 7.5 percent a year earlier.

Amy Martinez: 206-464-2923 or amartinez@seattletimes.com. On Twitter: @amyemartinez