A recent shakeup at the U.S. Department of Homeland Security (DHS) has raised concerns among lawmakers, including allies of U.S. President Donald Trump.

Senate Chuck Grassley, a Republican Senator from Iowa who previously served as Judiciary Committee Chairman, said in an interview with U.S. media Monday that he was "very concerned" over the possible ousting of more DHS senior officials, after Secretary of Homeland Security Kirstjen Nielsen and Director of the Secret Service Randolph Alles exited.

"They are the intellectual basis for what the president wants to accomplish in immigration," Grassley said, after rumors circulated Monday that Director of U.S. Citizenship and Immigration Services Lee Cissna, another DHS senior official, is in danger of being dismissed.

"The president has to have some stability and particularly with the number one issue that he's made for his campaign, throughout his two and a half years of presidency," Grassley said. "He's pulling the rug out from the very people that are trying to help him accomplish his goal."

Senator Ron Johnson, another Republican lawmaker who chairs the Senate Homeland Security and Governmental Affairs Committee, said in a statement Monday he was "concerned with a growing leadership void within the department tasked with addressing some of the most significant problems facing the nation."

It was not immediately clear why Nielsen and Alles exited, U.S. media widely attributed the shakeup to a push back from department officials in response to a White House drive to toughen border security.

The White House said earlier Monday that Alles "will be leaving shortly", while Trump announced Nielsen's exit in a tweet Sunday.

U.S. Customs and Border Protection Commissioner, Kevin McAleenan, has taken over the DHS as acting secretary, while James Murray, a career member of the Secret Service, will replace Alles.