Washington DC - More than one million undocumented migrants have left the US on their own since President Donald Trump took office, Homeland Security Secretary Kristi Noem said Friday.

Noem also said that hundreds of thousands of "criminal illegal aliens" have been arrested since January, and "zero illegal aliens" have entered the country during the past three months.

"[This] is the first time in this nation's history that we've seen that kind of security at our nation's border," she said at a press conference in Chicago.

"Our top priorities have been these two items: securing our borders and arresting dangerous criminals," she continued.

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"We've been working on getting dangerous criminal illegal aliens out of our country – murderers, rapists, drug traffickers, human traffickers."

Noem said thousands of migrants have "self-deported" using an app created by the Department of Homeland Security.

"But we also know that hundreds of thousands of people have left on their own, not utilizing a government program to do so," she said.

"We believe over a million people have gone home on their own since we have started this administration."

Trump vowed during his White House campaign to deport millions of undocumented migrants and has moved to significantly expand the federal agency primarily responsible for doing so, Immigration and Customs Enforcement (ICE).

"President Trump allocated to us the resources to hire 10,000 new ICE officers," Noem said. "We have had that open for less than a week, and we have over 80,000 applicants for those jobs."

Noem also lashed out during her press conference at officials in Illinois and other Democratic-ruled states, accusing them of "obstructing" federal efforts to remove undocumented migrants.