US Homeland Security Secretary Kirstjen Nielsen, who oversaw President Donald Trump's bitterly contested immigration policies during her tumultuous 16-month tenure, resigned on Sunday amid a surge in the number of migrants at the border with Mexico.
A senior administration official said Trump asked for Nielsen's resignation and she gave it.
Trump, who has recently expressed growing anger about the situation at the border, said on Twitter: "Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service."
In another tweet, Trump said Kevin McAleenan, the current US Customs and Border Protection commissioner, would become acting DHS secretary.
In a tweet late Sunday, Nielsen said that she would stay on until Wednesday.
"I have agreed to stay on as Secretary through Wednesday, April 10th to assist with an orderly transition and ensure that key DHS missions are not impacted," she said.