WASHINGTON — The number of Americans living in extreme poverty has grown by more than one-third over the past decade, the Brookings Institution said in a study out Thursday.

Over the past decade, the number of US residents living below the federal poverty line -- defined in 2010 as having an income of $22,314 for a family of four -- has grown by 12.3 million, read the report from the prestigious think-tank.

The number of Americans living in poverty has reached "a historic high of 46.2 million," equivalent to more than 15 percent of the US population, the report said.

People in extreme-poverty neighborhoods -- where at least 40 percent of residents live below the poverty line -- increased by one-third from 2000 to 2009 though these increases were not even across the country, read the report.

Brookings, which based its study on 2000-2009 income data from the US Census Bureau, notes that the fall erases economic gains from the 1990s.

Concentrated poverty nearly doubled in Midwestern US cities like Detroit, Michigan and Dayton, Ohio between 2000 and 2009, and rose by one-third in southern US cities like Baton Rouge, Louisiana and Jackson, Mississippi.

"These trends suggest the strong economy of the late 1990s did not permanently resolve the challenge of concentrated poverty," Brookings wrote.

"The slower economic growth of the 2000s, followed by the worst downturn in decades, led to increases in neighborhoods of extreme poverty once again throughout the nation, particularly in suburban and small metropolitan communities and in the Midwest."

According to the report, government policies "that foster balanced and sustainable economic growth at the regional level, and that forge connections between growing clusters of low-income neighborhoods and regional economic opportunity, will be key to longer-term progress against concentrated disadvantage."

Separately, the Wall Street Journal reported Wednesday that as of August nearly 15 percent of Americans rely on food stamps -- weekly US government stipends -- for survival.

The Journal, using US Department of Agriculture data, said that more than 21 percent of residents in the southern state of Mississippi rely on food stamps, as well as one-fifth of New Mexico, Tennessee and Louisiana residents.