The governor of a US state is refusing to quit over a racist photo in his medical school yearbook, despite growing calls for him to resign.

Virginia governor Ralph Northam has now insisted it is not him pictured, despite originally apologising on Friday for the photograph that featured a man in blackface and a second person in a Ku Klux Klan hood and robe.

In a video he posted on Twitter, the Democrat said he could not “undo the harm my behaviour caused then and today.”

But on Saturday, Mr Northam said he was not in the picture after all, adding that he had not seen the photo before Friday because he had never bought a copy of the 1984 yearbook or been involved in its preparation 35 years ago.

“It has taken time for me to make sure that it’s not me, but I am convinced, I am convinced that I am not in that picture,” he told reporters during a press conference in Richmond.

Mr Northam, left, with his wife, Pam, during the news conference
Mr Northam, left, with his wife, Pam, during the news conference (Steve Helber/AP)

Calling the picture offensive and horrific, Mr Northam, who is one year into his four-year term, again rejected demands that he step down.

While talking with reporters, the governor admitted he once used shoe polish to put on blackface as part of a Michael Jackson costume for a 1984 dance contest in Texas, while he was in the Army.

He said he regrets that he did not understand “the harmful legacy of an action like that”.

Asked by a reporter if he could still do Jackson’s famous moonwalk, Mr Northam paused before his wife Pamela interjected to say it was “inappropriate circumstances”.

The governor’s shifting explanations did little to sway prominent Democrats calling on him to resign.

Both of Virginia’s US senators, Mark Warner and Tim Kaine, joined the dean of Virginia’s congressional delegation, Representative Bobby Scott, in saying they no longer believe Mr Northam can serve effectively.

But his refusal to step down could signal a potentially long and bruising fight between Mr Northam and virtually all of the state’s Democratic establishment. Top Republicans in the Virginia General Assembly also urged him to quit.

Mr Northam was pushed repeatedly by reporters to explain why he had issued an apology if he was not in the photograph, and he conceded that people might have difficulty believing his shifting statements.

“My first intention … was to reach out and apologise,” he said, adding that he recognised that people would be offended by the photo. But after studying the picture and consulting with classmates, Mr Northam said: “I am convinced that is not my picture.”

The Eastern Virginia Medical School yearbook images were first published on Friday by the conservative news outlet Big League Politics.

The governor, a paediatric neurologist, has recently come under fire from Republicans who have accused him of backing infanticide after he said he supported a bill loosening restrictions on late-term abortions.

In a tweet on Saturday, President Donald Trump weighed in on proceedings, calling Mr Northam’s actions related to the photo and abortion debate “unforgivable!”