President Donald Trump has tweeted that US Homeland Security chief Kirstjen Nielsen is leaving her position as his administration pursues an overhaul of the department responsible for immigration.

Ms Nielsen’s departure was first reported by CBS News, which said it was unclear whether the resignation would be voluntary.

In another tweet on Sunday, Mr Trump said Kevin McAleenan, the Customs and Border Protection commissioner, would become acting Department of Homeland Security secretary.

….I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSgov. I have confidence that Kevin will do a great job!

— Donald J. Trump (@realDonaldTrump) April 7, 2019

Ms Nielsen, 46, has held the DHS position since December 2017.

The DHS’ responsibilities include immigration and customs enforcement. It is also responsible for carrying out some of Mr Trump’s most contentious policies as he has sought to cut back on the number of Mexicans and Central Americans entering the US without proper documentation.

Repeatedly subjected to tough questioning by Democrats in Congress, Ms 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.

Two Associated Press sources said Ms Nielsen went to the White House to speak with Mr Trump following their trip to the border late last week.

The sources were quoted as saying she was frustrated by difficulty getting other departments to help with the growing number of families coming across the border.

Mr Trump insists the arrival of immigrants along the southern US 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.”

-with AAP