Silicon Valley billionaire Elon Musk laid into the US Securities and Exchange Commission again today.

This comes after the SEC accused the Tesla chief executive of violating the terms of a settlement last year of fraud charges against him.

In a filing yesterday, the SEC said that in a tweet on February19 that Musk had broken a promise made last year to have his public statements vetted by the company's board, part of the deal. 

The regulator did not say what remedy it wanted the court to impose.

But the commitment was part of a settlement that headed off demands from the SEC for Musk to resign as Tesla CEO, seen by investors in the company as a substantial risk to its future. 

Musk, who initially accused the SEC of failing to read the company's annual reports, followed up with another tweet in the early hours of today. 

"Something is broken with SEC oversight," he wrote. 

Shares of the electric carmaker dipped 2.6% to $291 in trading before the bell, signaling concern among investors that Musk had reopened his feud with the regulator.

JP Morgan Securities analyst Ryan Brinkman said that in a worst case scenario, the SEC could again seek Musk's removal as CEO for violating the terms of the agreement and its shares could fall by a fifth, or more than $50, in value. 

"It is difficult to judge the likelihood of the reappearance of this worst case scenario, but on the other hand the current allegations seem much less serious than last year's," Brinkman told clients in a note. 

"If the SEC were to seek Mr. Musk's removal (perhaps subject to yet another settlement), we believe the shares may approach the mid-$200 levels."

Tesla's general counsel Dane Butswinkas, hired as an outside counsel to help settle the case with the SEC, resigned a day after Musk made the tweet to which the new SEC case refers.

The tweet read: "Tesla made 0 cars in 2011, but will make around 500k in 2019," an inaccurate claim which the SEC said had not been pre-approved by the board and was disseminated to over 24 million people.

Musk corrected his tweet four hours later to say that the "annualised production rate" at year-end 2019 would probably be about 500,000, with deliveries expected to be about 400,000. 

One analyst said that Musk's unwillingness to follow the rules is part of what you have to be willing to accept as an investor in Tesla.