Musk, CEO of electric car manufacturer Tesla and senior advisor to the president, says that Peter Navarro is "dumber than a sack of bricks" (approximately "dumber than the train") on social media site X, previously Twitter.
The background to the attack is an interview that Navarro gave with CNBC, where he described Musk as a "car assembler" – not a car manufacturer.
"Navarro is really a moron. What he says is demonstrably false", writes Musk.
Musk has deviated somewhat from Trump's protectionist line in recent days and has expressed hope for free trade between the US and EU. The South African tech billionaire has previously questioned Navarro's education at Harvard University.
The White House press secretary brushes off questions about Musk's attack.
Who cares? We are the most transparent government ever and express our differences openly, she says according to CNBC.
