Donald Trump launched an astonishing tirade at a leading black congressman on Saturday, calling him a "brutal bully" and the Maryland district he represents a "rodent infested mess".

The president's attack against Rep. Elijah Cummings seemed centred on what he perceived to be hypocrisy from the congressman over his criticism of poor conditions at migrant detention centres.

But his Twitter storm then took on a more personal approach in an apparent parroting of a 'Fox & Friends' segment that had aired earlier. Speaker Nancy Pelosi called his comments racist.

"Rep, Elijah Cummings has been a brutal bully, shouting and screaming... about conditions at the Southern Border when actually his Baltimore district is FAR WORSE," Mr Trump tweeted.

"Cumming District is a disgusting, rat and rodent infested mess. If he spent more time in Baltimore, maybe he could help clean up this very dangerous & filthy place," he added in a subsequent post.