UPDATE 2-U.S. Homeland Security Secretary Nielsen leaving her position

U.S. Homeland Security Secretary Kirstjen Nielsen is leaving her position, President Donald Trump said in a tweet on Sunday, as the administration pursues an overhaul of the department responsible for immigration policy.

Her departure was first reported by CBS News, which said it was unclear whether Nielsen's departure would be voluntary. In another tweet, Trump said Kevin McAleenan, the current U.S. Customs and Border Protection commissioner, would become acting DHS secretary.

Nielsen, 46, has held the position since December 2017. As it includes immigration and customs enforcement, her department is responsible for carrying out some of Trump's most contentious policies as he has sought to cut back on the number of Mexicans and Central Americans entering the United States without proper documentation. Repeatedly subjected to tough questioning by Democrats in Congress, Nielsen has overseen Trump plans such as his effort to build a wall on the border with Mexico and the separation of migrant children from their families.

Trump insists that the arrival of immigrants across the southern U.S. border constitutes a national emergency. He recently threatened to close the border, or parts of it, if Congress did not change U.S. laws to fix what he called immigration "loopholes."

(Reporting by Diane Bartz and Patricia Zengerle; Editing by Peter Cooney and Sonya Hepinstall)

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