A one-time classmate of President Donald Trump's Supreme Court nominee said Monday that Brett Kavanaugh was a habitual heavy drinker, challenging the judge's Senate testimony to the contrary.

The comments came days after the FBI opened a Senate-requested probe into allegations by three women that a drunken Kavanaugh sexually abused them or engaged in sexual misconduct while they were students in the 1980s.

"I can unequivocally say that in denying the possibility that he ever blacked out from drinking and in downplaying the degree and frequency of his drinking, Brett has not told the truth," Chad Ludington told reporters.

The North Carolina State University professor, who said he had contacted the FBI with his information, indicated in a statement Sunday that Kavanaugh was "belligerent and aggressive" when he drank.

Earlier, Trump had defended his pick but admitted that the appeals court judge has had a "bit of difficulty" with alcohol.

In a White House press conference, the US president sought to excuse excessive drinking by teenagers, while going beyond Kavanaugh's own testimony on his past use of alcohol to the Senate Judiciary Committee last week.

"I was surprised at how vocal he was about the fact that he likes beer," the president said.

"He's had a little bit of difficulty. He talked about things that happened when he drank. This is not a man that said... he was perfect with respect to alcohol."

He also questioned why investigators needed to examine the 53-year-old Kavanaugh's high school record.

"I think it's very unfair to bring up things like this," Trump said.

"I graduated from high school and while I did not drink, I saw a lot of people drinking," he said.

"They drink beer and go crazy and they were in high school... Does that mean that they can't do something they want to do in their life?"

He returned to the theme during a campaign rally in Tennessee, where he accused opposition Democrats of being motivated by politics.