Donald Trump has confirmed immigrants will need to leave the US if they wish to become citizens, under his immigration plans.

The GOP candidate unveiled plans last week that included his previous campaign promises to build a wall on the country's southern border and end immigration amnesties that allow previously undocumented workers to remain in the country.

"To become a citizen you're going to have to go out and come back in, through the process, you're going to have to get online," Trump said in an interview with Fox News. "This isn't touchback, you have to go out and come back in to become a citizen."

Reiterating his tough line on immigration, despite asserting prior to his long-awaited speech on the topic that he had softened his views, Trump said people would be made leave the country immediately if he wins the November election.

"They're going out so fast your head will spin, ok?" he told the news channel. "By the way that's a huge number, it's not a small number it's a tremendous number."

US authorities estimate there are up to 11m undocumented immigrants living in the country, the majority believed to hail from Mexico.

Despite Trump meeting with Mexican president Enrique Peña Nieto ahead of his immigration speech, he maintained there would be a wall between the two countries if he reached the White House, prompting Peña Nieto to reiterate that he would not permit Mexico to pay for such infrastructure.