The number of illegal immigrants crossing into the United States has reached the 2.5-million mark, according to the Center for Migration Studies and the Pew Research Center.

Of the total estimated number, 790,000 came in since 2013, the Washington Examiner reported.

From 2009 to 2013, 1.5 to 1.7 million people illegally entered US territory either through overstaying a temporary visa or by sneaking into the country, the Center for Immigration Studies reported.

Also, the Center said an additional 790,000 illegal migrants entered the country from mid-2013 to May this year, based on an analysis of Census Bureau data.

The figure increases the total to 2.5 million new illegal immigrants since Obama took office in January 2009, or 300,000 to 400,000 new illegal immigrants per year.

The overall population of immigrants, however, remained "roughly constant" as the wave of immigrants entering the US is balanced by those who return home or obtain legal status.

"Had the United States not allowed so many new illegal immigrants to settle in the country since 2009, the total number of illegal immigrants would have fallen by 2.5 million. But the arrival of so many new illegal immigrants offset this attrition in the illegal population," the Center for Immigration Studies reported.

Immigration has become a hot-button topic under the Obama administration as the additional illegal immigrants entered the US after Obama launched a contentious executive order back in 2012 to halt the deportation of young illegal immigrants who entered the country as children.

With the upcoming presidential elections, Republicans are especially worried about their image as one of their own candidates, Donald Trump, has slammed Mexico and its immigrants.

But a report by the Center said illegal immigration was actually higher under the Bush administration, which could possibly undermine the Republicans' attempt to sound tough on the issue.

The number of illegal immigrants sky-rocketed to 500,000 to 600,000 in a year under the Bush administration.