The number of illegal immigrants in the United Statesticked down slightly to 11.5 million as of January 2011, according to statistics released by the Obama administration on an issue likely to play a big role in the U.S. presidential campaign.

The U.S. Department of Homeland Security estimated the number has remained largely stable - just down from 11.6 million in 2010 - citing the high U.S.unemployment rate, improved economic conditions in Mexico and increased border enforcement.

"Mexico continued to be the leading source country of unauthorized immigration to theUnited States," according to the DHS report, noting that Mexicans made up 59 percent of the group, with 6.8 million believed to be in the country.

Illegal immigrants from El Salvador were a distant second, with some 660,000 believed to be in the United States.

States along the southwest border with Mexico topped the list with the most immigrants illegally in the country, with about 2.83 million in California, followed by almost 1.8 million in Texas, the DHS Office of Immigration Statistics report said.

Florida was a distant third with 740,000. The biggest percentage increase was inGeorgia from 220,000 in 2000 to 440,000 in January 2011, according to the report.

DHS based its new estimate on figures collected as part of the 2010 Census.

The issue of immigration has been a hot topic on the presidential campaign trail withMitt Romney, the leading candidate for the Republican nomination to face PresidentBarack Obama in November's election, backing tougher measures to crack down on illegal immigrants.

Obama, a Democrat, has sought to block some tough new state laws aimed at pushing out illegal immigrants and he has urged the U.S. Congress to pass a comprehensive package of immigration reforms, but that effort has gone nowhere.

Hispanic Americans are a growing voting bloc and both candidates will be vying for their support in November.