Elon Musk on Tuesday described Peter Navarro, a senior White House trade adviser, as "truly a moron" in a growing rift over Donald Trump's tariff policy.

Musk, a key aide to the president, has signalled his opposition to the tariffs, and the Tesla CEO hit out after Navarro described his electric vehicle company as "not a car manufacturer" but "a car assembler."

Trump's new tariffs on imports have roiled markets as fears of a global recession rise.

Musk posted his criticism on X under a clip of Navarro saying Tesla imported batteries, electronics and tyres, and that Musk "wants the cheap foreign parts."

Musk fired back, saying that "Tesla has the most American-made cars. Navarro is dumber than a sack of bricks."

The world's richest man recently backed 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 Republican Party. (AFP)