Washington - A sharp drop in the number of unauthorized immigrants in the United States that accompanied the start of the Great Recession has leveled off in recent years and could be ticking back upward as the economy improves, according to a report released Monday.

In all, the analysis from the Pew Research Center estimated that there were 11.7 million immigrants living in the country illegally in March 2012. That was down from an all-time high of 12.2 million in 2007, a year before the stock market collapsed, but it represented a slight increase from the estimated 11.3 million in 2009, during the worst year of the recession.

Analysts cautioned that the changes over the past few years could not definitively prove that the number of unauthorized immigrants is rising again because the figures represent the midpoint of ranges based on U.S. census surveys and other data.

"The 2012 population estimate is ... in a statistical sense is no different from the 2009 estimate," the authors of the report wrote. However, they said there is little doubt that the decline of unauthorized immigrants during the recession has "bottomed out."

The report's findings could lend new urgency to the debate over comprehensive immigration reform in Congress, as lawmakers wrestle with how to address the fate of the undocumented population. The Democratic-controlled Senate has approved a sweeping plan to add new layers of security along the U.S.-Mexico border, provide additional legal channels for immigration and allow those here illegally to gain citizenship over 13 years if they pay fines, learn English, remain employed and do not commit crimes.

But the Republican-led House has shelved that plan and focused instead on a series of piecemeal bills that do not include measures to legalize undocumented immigrants. The Pew study builds on the organization's work examining immigration trends, but it does not recommend a path forward on legislation.

The number of people living in the United States illegally has skyrocketed since 1990, when there were an estimated 3.5 million unauthorized immigrants. A sweeping overhaul of immigration laws in 1986, which granted about 2.5 million people a path to citizenship, failed to stem the flow of immigrants who came to the United States illegally to look for jobs or to join family members already here.

Previous Pew studies have shown that the net flow of immigrants across the U.S-Mexico border has shrunk dramatically in recent years, falling to nearly zero as the Obama administration, building on programs implemented under President George W. Bush, ramped up enforcement measures that have led to record numbers of deportations. Last year, Immigration and Customs Enforcement deported nearly 410,000 people, prompting criticism from advocates who say immigrants are living in fear and being separated from their families.

The Obama administration has hailed the deportation figures as evidence that the enforcement programs are working, but Senate negotiators included billions of dollars for new border security measures in their immigration bill to woo Republicans. The legislation passed in June on a vote of 68-32.

But groups opposed to allowing unauthorized immigrants legal status have argued that the drop in the population has been driven primarily by economic factors, as the U.S. economy hurtled into a recession and the Mexican economy improved in recent years.

The Pew study found that the number of Mexican immigrants living illegally in the United States has dropped steadily over the past five years to just over 6 million, down from a high of 6.9 million in 2007, and the lowest amount since 2004. By contrast, the number of unauthorized immigrants from other nations has reached an all-time high of 5.7 million, the study concluded.

"Because of the decline in Mexicans and growth in unauthorized immigrants from other countries of origin, Mexicans represented a smaller share of unauthorized immigrants in 2012 (52 percent) than in 2007 (57 percent)," the analysts wrote.

They also examined trends in six of the states with the highest number of unauthorized immigrants: California, Florida, Illinois, New Jersey, New York and Texas. Of those, only Texas has seen a steady rise in the undocumented population, with a total of 1.7 million living in the state illegally in 2012, about 200,000 more than in 2007.

Texas Sens. John Cornyn and Ted Cruz, both Republicans, voted against the Senate's immigration plan, while Arizona Sens. John McCain and Jeff Flake, also Republicans who represent a border state, were among the group that authored the legislation and voted in favor of it.