A US Secret Service agent assigned to protect Vice-President JD Vance has been suspended and is under investigation over allegations of leaking sensitive information, the agency said on Thursday.

The specifics of what was leaked have not been released, but the USSS said the agent faced a "potential criminal inquiry", involving allegations of "compromising operational and information security".

"Any conduct that potentially threatens the safety of a protectee will not be tolerated," USSS spokesperson Anthony Guglielmi said.

This comes as agency officials warn that threats against its protectees, including President Donald Trump, have seen a sharp rise in numbers.

The White House declined to comment on the investigation.

Vance and his family are required to have round-the-clock Secret Service protection.

On Wednesday, Secret Service Director Sean Curran said the agency was operating in what he described as the most dangerous threat environment of his 24-year career, citing a sharp rise in threats against public officials.

Speaking to reporters ahead of Friday's rescheduled White House Correspondents' Dinner, Curran said the threat level facing the Secret Service is "off the charts".

"The environment has become very volatile, and that's just across the board," Curran said. "The threat picture and environment is as large as I've ever seen it."

Agents have opened nearly 10,000 threat investigations since January.

Officials said the increase was being driven by a mix of online threats, lone actors, ideological extremism, foreign adversaries and individuals experiencing mental health crises.

They added that most threats now emerge online, replacing handwritten letters and other traditional forms of communication as the primary source.