WASHINGTON, March 23: The undocumented population of the United States has increased to nearly 11 million people despite strict anti-immigration measures introduced after the Sept 9, 2001, attacks, says a report released on Monday. More than six million of these illegal immigrants are from Mexico, which borders America.

The estimates, collected by Washington’s Pew Hispanic Center and based on the most recent official data, show an increase of about 23 per cent in the population of undocumented residents, from 8.4 million to 11 million in the four-year period ending last March.

In the southern states, where the immigrant population is booming, those numbers are even higher. There has been a nearly 63 per cent increase in immigrant population in Alabama, 58 per cent in North Carolina and 55 per cent in Georgia.

Nearly a quarter of the immigrant population lives in California.

The Pew center says the prospect of better job opportunities in the United States remains a powerful lure for many immigrants.