US President Barack Obama has denounced Republican presidential candidate Donald Trump’s plan to use a “deportation force” to deport millions of undocumented immigrants as unrealistic.

“The notion that we’re gonna deport 11, 12 million people from this country — first of all, I have no idea where Mr. Trump thinks the money’s gonna come from," Obama said on Thursday in an interview with ABC News.  "It would cost us hundreds of billions of dollars to execute that."

“Imagine the images on the screen flashed around the world as we were dragging parents away from their children, and putting them in what, detention centers, and then systematically sending them out,” he said.

“Nobody thinks that that is realistic. But more importantly, that’s not who we are as Americans,” the US president stated.

On Wednesday, Trump told MSNBC he would deport 11 million undocumented workers from the United States and would establish a "deportation force" for this purpose. "You're going to have a deportation force, and you're going to do it humanely.”

Trump, who has never held elected office, is still leading the crowded Republican presidential primary field, despite the fact that his campaign has been marked by controversial statements, including with disparaging remarks about Mexican immigrants.

Trump has promised, if he wins the White House in 2016, to expel undocumented immigrants and build a wall on the US-Mexico border.

He has also proposed repealing the constitutional right to citizenship of anyone born on US soil.

Trump’s fiery remarks on immigration and other issues have dominated the 2016 election campaign, and during the fourth GOP debate on Tuesday night the billionaire businessman renewed his calls for mass deportations, drawing rebukes from several of his rivals.  

Ohio Governor John Kasich termed Trump’s plan as “silly” and former Florida Governor Jeb Bush said this could scare off Latino voters from supporting Republicans.