Multiple reports indicate the White House adviser Stephen Miller has orchestrated a hardline shake-up of the Department of Homeland Security.

Kirstjen Nielsen announced her resignation as secretary of homeland security on Sunday, with President Donald Trump reportedly concerned that she had not taken tough enough stances.

The Washington Post reported Friday that the president recently told Miller he was in charge of all immigration and border affairs.

With illegal border crossings at an 11-year high in February, the Trump administration has renewed attempts to crack down on migration from Mexico.

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US border and immigration policy is now said to be under the control of one of the most hardline anti-immigration members of the Trump administration, following the weekend's abrupt resignation of Homeland Security Secretary Kirstjen Nielsen.

On Sunday, Politico reported that the White House adviser Stephen 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 illegal movement of migrants into the country.

"There's definitely a larger shake-up 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 a planned overhaul of the Department of Homeland Security, where President Trump reportedly didn't believe the outgoing Nielsen shared his anti-migration stance.

Trump visiting a new section of the border wall with Mexico in Calexico, California, on Friday. Associated Press

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

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

According to administration officials cited in The Washington Post, Miller had 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.

Miller seen as Trump hosted a discussion on border security and safe communities with state, local, and community leaders in the White House Cabinet Room on January 11. Photo by Alex Wong/Getty Images

The president even told Miller recently that he would be in charge of handling all immigration and border affairs, The Post said Friday.

The outlet said Miller had recently encouraged the president to take a more confrontational approach with Mexico and supported his threat to close the legal ports of entry on 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, which received widespread international and domestic condemnation, saw children who had entered the US illegally separated from their parents at the border and held in detention facilities.

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 illegal immigration.

Miller said the White House was "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."