President Donald Trump said Sunday that he has accepted the resignation of Kirstjen Nielsen as head of the US Department of Homeland Security (DHS).

Nielsen's resignation came after Trump voiced frustration with an increase in the number of migrants crossing into the US from Mexico.

Read more: Trump threatens to close US-Mexican border

What you need to know:

• Nielsen confirmed her resignation on Twitter after Trump's announcement. She did not say why she was leaving.
• Kevin McAleenan, the head of US Customs and Border Protection, will become acting DHS secretary, he added.
• Nielsen was frustrated with other departments for not helping the DHS enough in managing migrant families caught on the the southern border, according to the Associated Press.

'Honor of a lifetime'

Nielsen said: "Its been an honor of a lifetime to serve with the brave men and women of @DHSgov. I could not be prouder of and more humbled by their service, dedication, and commitment to keep our country safe from all threats and hazards."

Awkward numbers: Nielsen's future as DHS secretary had been in doubt amid surging numbers of migrants entering the United States. Some 100,000 migrants were caught at the border with Mexico in March, the highest level in a decade, according to US border officials.

Who is Kirstjen Nielsen? The 46-year-old became assistant DHS secretary in January 2017. She then had a spell as deputy White House chief of staff from September 2017 until she became DHS chief in December 2017.

What does the DHS do? Former US President George Bush set up the department in the aftermath of the September 11, 2001 al-Qaida attacks to help the US fight terrorism and ensure border security.

amp/ls (AFP, dpa, AP, Reuters)