President Donald Trump's legal team was said to have been threatened with a grand-jury subpoena should the president refuse to sit down with investigators in the Russia inquiry.

People close to Trump say he's angry about it and spoiling for a fight.

"For the average human, nothing scares them more than legal issues," the news website Axios quoted a source close to Trump as saying. "He. Does. Not. Care. His whole adult life has been spent in litigation. He's not afraid of high-stakes legal stuff … He's just going to start swinging and knock people's heads off."

But a legal expert told Axios he didn't think Trump would be able to take the intense questioning Mueller would put him through.

President Donald Trump has made no secret of his contempt for the special counsel Robert Mueller's Russia investigation — and people close to the president now say he's on a warpath.

Mueller has reportedly threatened Trump's legal team with a grand-jury subpoena should Trump refuse to sit down with investigators in the inquiry into whether his campaign colluded with Russia to influence the 2016 US election and whether he attempted to obstruct justice.

And experts don't think it would go well for the president.

Trump's "instinct is always to be on the offensive," a source told the news website Axios. "This was a wake-up call to the president that will embolden him, in a lot of ways. As he sees this becoming more serious, his instinct is to punch back 10 times harder."

The White House has suggested that Trump believes he has the power to dismiss Mueller. But critics have said that such a move could lead to what they have described as a "constitutional crisis."

On Twitter, Trump has repeatedly criticized the investigation as a "witch hunt" and cast the spotlight on his political opponents.

Axios quoted another source "very close to Trump" as saying: "For the average human, nothing scares them more than legal issues. He. Does. Not. Care. His whole adult life has been spent in litigation. He's not afraid of high-stakes legal stuff … He's just going to start swinging and knock people's heads off."

On Tuesday, Trump attacked news reports detailing what Mueller would like to ask him, bizarrely claiming that the questions didn't address collusion, though about a dozen did.

The reported questions would leave little room to hide, and any misleading or untruthful statements by Trump could lead to prosecution — most likely compounding the president's anger and propensity to make counteraccusations.

"I do not see him sustaining the hours of questioning he would face," Bob Bauer, a White House counsel under President Bill Clinton, told Axios. "And to some questions — such as his role in the Trump Tower matter and the fake press release he directed for Don Jr. — he seems unlikely to have good answers."