A photograph of former presidential candidate Hillary Clinton looking at a newspaper headline about vice-president Mike Pence using private email has gone viral on social media, with thousands of people commenting on it.

On Friday, Clinton was on a plane traveling from Boston to New York when a fellow passenger snapped a photo of her glancing down at Friday's USA Today front page headline: "Pence used personal email in office."

Pence and others involved in the Republican presidential campaign last year criticized Clintons use of a private email server during her time as secretary of state.

Donald Trump and his running mate regularly said Clinton broke the law and endangered national security, complaints that led supporters at rallies to chant "Lock her up!"

Trump also threatened to send Clinton to jail if he won, a threat he has subsequently dropped.