Billionaire Elon Musk blasted President Donald Trump's senior trade advisor Peter Navarro as "truly a moron" and "dumber than a sack of bricks" on Tuesday in a growing rift over the US tariff policy that has rocked the world.

The extraordinary public spat came after Navarro described the Tesla boss and so-called Department of Government Efficiency (DOGE) chief as "not a car manufacturer" but "a car assembler" who relies on imported parts.

Musk, the world's richest person, has previously signaled his opposition to the president's new import tariffs that have roiled markets.

"Navarro is truly a moron. What he says here is demonstrably false," Musk posted on his X social network, under a clip of Navarro saying Tesla imported batteries, electronics and tires, and that Musk "wants the cheap foreign parts."

Musk doubled down in a series of other messages, saying that "Tesla has the most American-made cars. Navarro is dumber than a sack of bricks."

Musk also dubbed him "Peter Retarrdo" and said Navarro "should ask the fake expert he invented, Ron Vara" -- referring to a fictional pundit Navarro quoted in a series of books and a policy memo, using an anagram of his own name.

The South African-born tycoon recently backed the idea of a free-trade zone between North America and Europe -- a wish at odds with Trump's flagship tariffs.

The US president has ruled out any pause in his aggressive stance despite retaliatory action from China and signs of criticism from within his normally loyal Republican Party.

'Car assembler'

But there have also been conflicting messages from within the White House itself. 

A long-time China hawk, Navarro has been one of the most hardline voices on tariffs, and targeted Musk himself in an interview with CNBC.

"When it comes to tariffs and trade, we all understand in the White House, and the American people understand, that Elon's a car manufacturer. But he's not a car manufacturer -- he's a car assembler in many cases," Navarro said.

"If you go to his Texas plant... the batteries come from Japan and from China, the electronics come from Taiwan."

The row came a day after Navarro insisted in an opinion piece in the Financial Times that the tariffs were "not a negotiation" — only for Trump to admit later that he was in fact open to some negotiations.

The spat is all the more unusual because of the mesh of loyalties involved.

Trump has strongly defended Musk after a series of vandalism attacks and protests against Tesla over DOGE's cost-cutting drive -- even turning the White House into a pop-up showroom for the electric vehicles in a show of support.

Navarro, however, has proven his loyalty to Trump by serving a four-month jail sentence for contempt after refusing to testify to Congress on the 6 January 2021 attack on the US Capitol by Trump supporters.