U.S. Republican presidential nominee Donald Trump said on Saturday he would seek to develop a tracking system to ensure visitors who overstay their visas and become illegal immigrants are removed from the United States.

Under fire for giving mixed messages on his signature issue, illegal immigration, Trump used a campaign speech in Des Moines, Iowa, to provide some fresh details.

"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," Trump said.

"If we don't enforce visa expiration dates, then we have an open border, it's as simple as that."