US Homeland Security Secretary Kirstjen Nielsen, who oversaw President Donald Trump's bitterly contested immigration policies during her tumultuous 16-month tenure, has resigned amid a surge in the number of migrants at the border with Mexico.
A senior administration official said Mr Trump asked for Ms Nielsen's resignation and she gave it.
Mr Trump has recently expressed growing anger about the situation at the border.
Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service....
— Donald J. Trump (@realDonaldTrump) April 7, 2019
In another tweet, Mr Trump said Kevin McAleenan, the current US Customs and Border Protection commissioner, would become acting DHS secretary.
In a tweet late last night, Ms 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.
Ms Nielsen's, whose departure was first reported by CBS News, had been DHS secretary since December 2017.
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.
— Sec. Kirstjen Nielsen (@SecNielsen) April 8, 2019
Her departure had been repeatedly rumoured over the past year, particularly after a wave of anger over the administration's 2018 family separation policy at the border with Mexico.
Most recently, there was speculation she could leave her role as US border officials estimated that 100,000 migrants were apprehended at the southern border in March, the highest level in a decade.
Another senior administration official said Mr Trump's national security adviser, John Bolton, after a blowup with Ms Nielsen late last year, also recommended to Mr Trump that she should go.
Mr Trump has made a clampdown on illegal immigration a centrepiece of his two-year-old presidency, leading chants of "build that wall" at his rallies as he has sought to cut back on the number of newcomers entering the US without proper documentation.
Many of the migrants picked up last month were Central Americans seeking US asylum.
Mr Trump was so frustrated about the increase that he announced he would cut off US aid to Guatemala, Honduras and El Salvador.
He also threatened to close the border with Mexico, although he later backed off that proposal with a threat to impose tariffs on auto imports.
In her resignation letter, Ms Nielsen asked for more from Congress and the courts, which have opposed such Trump administration initiatives as his effort to limit immigration from Muslim nations and the border wall.
"I hope that the next Secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America's borders and which have contributed to discord in our nation's discourse," she wrote to Mr Trump.
