Former New York City mayor Rudy Giuliani has been injured in a car accident in the US state of New Hampshire, his security guard says.

Giuliani's car was struck from behind at high speed while travelling on a highway, according to a statement posted on social media.

"He was diagnosed with a fractured thoracic vertebrae, multiple lacerations and contusions, as well as injuries to his left arm and lower leg," the statement from security guard Michael Ragusa adds.

Giuliani became known as "America's Mayor" after leading New York through 9/11. He later became an adviser and then personal lawyer to Donald Trump.

The incident happened shortly after Giuliani had helped an alleged victim of domestic violence who had flagged him down on a road, the statement says.