Republican presidential nominee Donald Trump has linked illegal immigration and employment, pledging to start deporting offenders as soon as he is sworn in, should he become the White House's next occupant.

"On day one, I am going to begin swiftly removing criminal illegal immigrants from this country — including removing the hundreds of thousands of criminal illegal immigrants that have been released into US communities under the Obama-Clinton administration," Mr Trump said in Des Moines, Iowa.

Mr Trump's Democratic rival Hillary Clinton served as secretary of state during President Barack Obama's first term in office.

The next president will be sworn in on January 20.

"I am going to build a great border wall, institute nationwide e-verify, stop illegal immigrants from accessing welfare and entitlements and develop an exit-entry tracking system to ensure those who overstay their visas are quickly removed," Mr Trump warned.

The billionaire real estate magnate and former reality TV host said "if we don't enforce visa expiration dates, then we have an open border. It's as simple as that".

"A vote for Trump is a vote to have a nation of laws, a vote for Clinton is a vote for open borders," he added.

Mr Trump also has sought to court black votes, promising to help African Americans find jobs.

"Every time an African-American citizen, or any citizen, loses their job to an illegal immigrant, the rights of that American citizen have been totally violated," he argued.

Details of Mr Trump's immigration policies remain scant.

He rallied much of his primary support with a controversial hardline tone against illegal immigrants and his plan to build a wall on the Mexican border.

Some of his advisors are now reportedly urging him to tone down his signature policy priority.

AFP