But the estimate of immigrants living in the country without legal permission as of March 2012 is still 500,000 less than 2007's 12.2 million peak, before the financial crisis, the non-partisan Pew Research Center Hispanic Trends Project study indicated.
The figures suggest the number of unauthorized immigrants is rising, but the 2012 population estimate, based on U.S. Census Bureau data, is statistically no different from 2009's 11.3 million estimate, the center said.
A more precise census measure will be released later this year.
However, other factors, such as an increase in U.S. border apprehensions, indicate the recessionary decline in illegal immigration bottomed out and could be rising, Pew said.
The number of immigrants apprehended without documents at the Mexican border rose to 388,422 in the 11 months that ended in August, up from 364,768 in fiscal 2012 and 340,252 the year before, U.S. Border Patrol figures indicate.
The Border Patrol's fiscal years run through September.
Most of the latest apprehensions are of non-Mexicans, the figures indicate. Apprehensions of Mexicans are on the decline.
Mexicans accounted for 52 percent of unauthorized immigrants in 2012, down from 57 percent in 2007.
From 2007 to 2009, more undocumented Mexicans left the United States than entered the country illegally, Pew's report said.
Of the immigrants living in the United States without legal permission, 60 percent live in six states -- California, Florida, Illinois, New Jersey, New York and Texas.
Of these, only Texas had increases but no decrease in its unauthorized population during the 2007-2011 period, Pew said.
California, Illinois and New York had only declines.
Pew said it would publish estimates for all 50 states and the District of Columbia in a later report.
Its report comes as lawmakers in Washington debate an immigration overhaul that could include a pathway to legal status or citizenship for millions of unauthorized immigrants.
The reported rise in apprehensions would likely be seen among Republicans as supporting calls to better secure the U.S. border with Mexico before Congress lets immigrants already in the country illegally gain legal status or citizenship, The Wall Street Journal said.
Some GOP lawmakers have called for aggressive approaches, such as doubling the number of agents or constructing a full border fence.
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