US President Donald Trump on Sunday announced the departure of Homeland Secretary.

Iran Press/America: US President Donald Trump underscored his intent to toughen immigration policy amid a surge in illegal crossings along the southern border after the resignation of Kirstjen Nielsen.

Homeland Secretary's exit marks the end of a difficult relationship with her boss, who was said to be unhappy with her performance despite her unswerving loyalty and full-throated defense of the president's most controversial policies, AFP reported.

"Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside," Nielsen said in a resignation letter she later shared on Twitter.

Donald Trump greets Kirstjen Nielsen in the White House, October 12, 2017. REUTERS

The move came just two days after she and Trump visited the Mexican border in California together, with the president delivering a stern message to would-be illegal immigrants and asylum seekers: "Our country is full."

During her 18 months at the helm of the powerful agency, the 46-year-old became synonymous with the controversial practice of separating children from their parents, making her a frequent target of progressive groups and the Democratic opposition who repeatedly called on her to resign.

None of this, however, seems to have been enough for Trump.

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

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

Despite warnings of dire economic consequences, including by top Republicans, Trump has repeatedly threatened to close the US-Mexico border, demanding that Congress and Central American governments act to stem a flow of migrants that saw Nielsen last week order an "emergency surge" of personnel to handle the situation.

This is while, House Democrats voted to authorize a lawsuit against the Trump administration’s emergency declaration. 101/201/213

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Donald Trump greets Kirstjen Nielsen in the White House, October 12, 2017. REUTERS