President Donald Trump has said he has accepted the resignation of Homeland Security Secretary Kirstjen Nielsen.
Mr Trump announced in a tweet: "Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service…."
In another tweet, the president said Kevin McAleenan, the current US Customs and Border Protection commissioner, would become acting head of the department.
Mr McAleenan is a long-time border officer, reflecting Mr Trump's priority for the department of 240,000 people, initially founded to combat terrorism after the September 11 attacks.
The move comes amid growing frustration by the Trump administration over the number of Mexicans and Central Americans entering the US without proper documentation.
Mr Trump has insisted that the arrival of immigrants across the southern border constitutes a national emergency.
He recently threatened to close the border, or parts of it, if Congress did not change US laws to fix what he called immigration "loopholes".
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Tensions between the White House and Ms Nielsen had been ongoing almost from the moment she became secretary in December 2017, after her predecessor John Kelly became the White House chief of staff.