WASHINGTON (AFP, REUTERS) - 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 (April 7) amid a surge in the number of migrants at the border with Mexico.

A senior administration official said Mr Trump asked for Nielsen's resignation and she gave it.

"Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service," Mr Trump tweeted.

He added that US Customs and Border Protection Commissioner Kevin McAleenan would become acting secretary.

Ms Nielsen's departure was first reported by CBS News.

In a tweet late Sunday, 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.

Her resignation was the latest high-profile departure from the Trump administration and leaves just four women in his Cabinet. Among others, Mr Trump currently lacks a permanent secretary of defence or chief of staff.

In her resignation letter, Ms Nielsen, 46, asked for more from Congress and the courts, which have opposed such Mr Trump administration initiatives as his effort to limit immigration from Muslim nations and the separation of children from their families.

"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.

Ms Nielsen initially joined the Trump administration in January 2017 as an assistant to Mr Trump's first Department of Homeland Security (DHS) secretary, Mr John Kelly. When Mr Kelly moved to the White House as Mr Trump's chief of staff in July 2017, Ms Nielsen went with him as his deputy.

But by October she was back at DHS, this time as secretary.

In that role, she has become the face of the Trump administration's anti-immigration policy, including the widely condemned practice of separating migrant children from their parents, as part of the administration's "zero tolerance" approach to illegal immigration, intended to deter families from leaving home in the hope of entering the US.

After domestic and international criticism, Mr Trump signed an executive order in June ending family separations, but a government report last month showed that more than 200 children had been taken from their families since that time.

Ms Nielsen's relationship with Mr Trump has long been said to be difficult. But despite reports that he complained constantly about her performance, she remained steadfastly loyal.

Last month, she defended the President's declaration of a national emergency to secure funding for his pet project: a wall on the US-Mexico border.

Her departure has been rumoured repeatedly.

News reports said Mr Trump had belittled her in Cabinet meetings, unhappy with her efforts to tighten immigration.

The New York Times reported nearly a year ago, in May 2018, that Ms Nielsen was close to resigning after Mr Trump berated her in front of other Cabinet secretaries over increases in the number of immigrants crossing illegally from Mexico.

Representative Bennie Thompson, the Democratic chairman of the House of Representatives Homeland Security Committee, said Ms Nielsen's tenure at DHS "was a disaster from the start".

He said in a statement, however, that she should not serve as a scapegoat, blasting Mr Trump for "terrible and cruel policies".

Noting that the department now has neither a permanent secretary nor deputy secretary, he called on the administration to work with Congress in good faith to keep the country safe.