PORTLAND — Maine’s urban areas showed robust population growth over the past decade while the state’s rural regions had slow growth or shrunk, according to population figures released Thursday by the U.S. Census Bureau.

The state’s three most populous counties — Cumberland, Penobscot and York — had population gains of 5.6 to 6.2 percent for the decade, Census Bureau statistics show. As a whole, the state grew by 4.2 percent, to more than 1.3 million people.

Maine’s eastern- and northern-most counties, which are among the most rural in the state, lost population. Washington County in far eastern Maine lost 3.2 percent of its people from 2000 to 2010. Aroostook County in far northern Maine saw its population fall 2.8 percent.

The primary reason for the sluggish showing in rural areas and strong growth in urban areas was the pressure to relocate in search of jobs as paper mills and other manufacturing plants have shut down or cut back, said Charles Colgan of the University of Southern Maine’s Muskie Institute of Public Service.

“The simple answer is the rural economy is stagnant or shrinking and the urban areas are growing,” Colgan said. “The net result is people are moving from the rural areas to urban areas.”

For the decade, the state’s metropolitan areas grew at a healthy pace.

The state’s largest city, Portland, grew by 3 percent to 66,194 while the surrounding suburbs also had strong gains. The population grew by 7.2 percent in South Portland, by 11.5 percent in Scarborough, by 14.1 percent in Windham and 15.8 percent in Gorham.

“The Portland area has real strength in terms of center city growth and in the suburbs,” Colgan said. “That runs counter to the trend of the past 30 years when center cities were stagnant and suburbs were growing. Now both are growing.”

The population in Lewiston, the state’s second-largest city, grew by 2.5 percent. Bangor, the third-biggest city, grew by 5 percent while Bangor suburb Orono grew at 13.7 percent clip.

Maine’s minority population is also growing.

In 2010, minorities made up about 5 percent of the state’s population, up from 3.1 percent in 2000. While the numbers are small, the gains are significant, Colgan said.

“When you consider that in the 1990 census we were only 1 percent minorities and now we’re nearly 5 percent minorities, that’s a noticeable change,” he said.

Maine’s black population grew the fastest — 128 percent — among the state’s minority groups, to 21,764, according to the Census. The increase reflects the influx of immigrants, particularly from Sudan and Somalia.