Before Nielsen was abruptly fired, hardline Trump adviser Stephen Miller was reportedly handed total control of border policy

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Stephen Miller talks to reporters about President Donald Trump's immigration system in December 2017.

• According to multiple reports, White House adviser Stephen Miller orchestrated a hardline shakeup of the Department of Homeland Security (DHS).
• DHS secretary Kirsten Nielsen unexpectedly announced her resignation Sunday, with President Trump reportedly concerned that she had not taken tough enough stances.
• The president, in a meeting of officials in the Oval Office last week, told Miller he was in charge of all immigration and border affairs, reported the Washington Post.
• With border crossings at an 11-year high, the Trump administration has renewed attempts to crack down on migration from Mexico.

US border and immigration policy is now reportedly under the control of one of the most notorious anti-immigration hardliners in the Trump administration, following the surprise departure of Homeland Security Secretary Kirsten Nielsen.

On Sunday Politico reported that Miller was behind a government-wide bid to tighten migration policy. The site said he lobbied for the replacement of government officials with those who share his views, and telephoned mid-ranking officials at several government departments to angrily demand that they do more the halt the movement of illegal migrants into the country.

"There's definitely a larger shakeup abreast being led by Stephen Miller and the staunch right wing within the administration," a person close to Nielsen told the publication. "They failed with the courts and with Congress and now they're eating their own."

CBS News also reported that Miller was behind the planned overhaul of the Department of Homeland Security (DHS), where Trump reportedly didn't believe outgoing secretary Nielsen shared his anti-migration stance.

With border crossings at an 11-year high, the Trump administration is doubling down on its pledge to stop migrants crossing the United States border without permission.

Nielsen's resignation came two days after Trump unexpectedly withdrew his own nomination to head the Immigration and Customs Agency (ICE), Ronald Vitiello, claiming he wanted someone "tougher" in the role.

Miller has, according to administration officials cited in the Washington Post, been a staunch critic of the nominee, with Trump's decision to ditch him seen as a sign of the 33-year-old adviser's expanding influence.

The president, in a recent Oval Office meeting, even told Miller he would be in charge of handling all immigration and border affairs, according to the Post.

The outlet said Miller had recently encouraged the president to take a more confrontational approach with Mexico, and supports his threat to close the US-Mexico border.

Miller has long been regarded as one of the strongest anti-immigration voices in the administration, and the architect of the controversial child separation policy.

The policy saw the children of undocumented migrants separated from their parents at the border and held in detention facilities, in a policy that received widespread international and domestic condemnation.

In an email last week to the conservative Daily Caller website, Miller said there would be an "aggressive effort to utilize every existing authority in statute" to curb undocumented migration.

Miller said the White House is "systematically reviewing all authorities that are already on the books, both in terms of cracking down on illegal immigration and […] the abuse of our legal immigration system."