WASHINGTON (Reuters) - President Donald Trump has ordered a shake-up of the leadership of the Department of Homeland Security, the head of the U.S. Secret Service said on Monday, after the White House said the official would be soon be leaving his post.

U.S. Secret Service Director Randolph Alles participates in a news conference about "significant law enforcement actions related to elder fraud" in Washington, U.S. March 7, 2019. REUTERS/Erin Scott

The Secret Service said Randolph “Tex” Alles would depart his job next month. The announcement came a day after Trump ousted Alles’ boss, Homeland Security Secretary Kirstjen Nielsen, with whom he had clashed over immigration issues.

“No doubt you have seen media reports regarding my ‘firing.’ I assure you that this is not the case, and in fact was told weeks ago by the administration that transitions in leadership should be expected across the Department of Homeland Security,” Alles said in a message to Secret Service agents.

Neither Trump nor the White House has explained the overhaul of DHS, but the president’s anger over a recent surge in migrants from Central America has been well documented. The DHS oversees immigration and border security.

The Republican president made stopping illegal immigration a centerpiece in his run for office in 2016, promising to build a wall on the southern border with Mexico. Trump has said he will make border security a key part of his campaign for re-election in 2020.

Senior senators from both parties said they were concerned about a vacuum in leadership at the agency, which also oversees the Secret Service, the Coast Guard, the Federal Emergency Management Agency and the Transportation Security Administration, among other critical functions.

Ron Johnson, the Republican chairman of the Senate Homeland Security Committee, said in a statement that he was “concerned with a growing leadership void within the department tasked with addressing some of the most significant problems facing the nation.”

Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, said at least 10 top positions in the department were being filled with acting officials.

“The purge of senior leadership at the Department of Homeland Security is unprecedented and a threat to our national security,” Feinstein said in a statement.

The Secret Service, which is responsible for protecting senior U.S. officials including the president and vice president, former presidents and visiting heads of state, came under scrutiny last week after a Chinese woman carrying electronic devices was charged with bluffing her way through security checks at Trump’s Mar-a-Lago resort in Florida.

White House spokeswoman Sarah Sanders said James Murray, a career Secret Service agent, would take over the agency in May.

Alles had run the Secret Service for two years and served in the U.S. Marine Corps for 35 years before retiring as a major general in 2011.

LONG LIST OF DEPARTURES

Alles was the latest in a long list of senior officials to leave the Trump administration, including the secretaries of state, defense, homeland security, interior, veterans affairs and health and human services, the attorney general and numerous senior White House aides.

The first sign of Trump’s overhaul of Homeland Security came late on Thursday, when he abruptly pulled his nomination of Ron Vitiello as director of U.S. Immigration and Customs Enforcement, part of DHS.

Trump addressed the decision perfunctorily to reporters as part of a policy change “going in a tougher direction.”

It was followed by the resignation of Nielsen on Sunday. A senior administration official said Trump asked Nielsen to step down. On Twitter, Trump said Kevin McAleenan, the U.S. Customs and Border Protection commissioner, would become acting DHS secretary.

CNN reported that L. Francis Cissna, director of the U.S. Citizenship and Immigration Services agency, and department General Counsel John Mitnick were also expected to leave their positions soon. The White House and DHS did not respond to requests for comment.

Trump in recent weeks empowered his hard-line conservative aide Stephen Miller to lead the administration’s border policies, CNN said.

Nielsen had been a trusted aide to John Kelly, Trump’s first DHS secretary and later his chief of staff, with whom he had an often-difficult relationship.

Nielsen and Trump had often clashed over immigration issues during her 16 months in the job. Her departure had been long rumored particularly after criticism of the administration’s 2018 family separation policy at the border with Mexico and as U.S. border officials estimated that 100,000 migrants were apprehended at the southern border in March, the highest level in a decade.

Trump was so frustrated about the surge in migrants that he announced he would cut off U.S. aid to Guatemala, Honduras and El Salvador.