US President Donald Trump speaks in the Oval Office of the White House in Washington.

US President Donald Trump was hit with new accusations of racism Saturday after he attacked a prominent African-American lawmaker and branded the majority black city of Baltimore an "infested mess."

Trump's outburst came in a series of sharply worded tweets aimed at Democratic Representative Elijah Cummings -- a high-profile critic of Trump's administration whose district covers much of Baltimore.

"Cumming (sic) District is a disgusting, rat and rodent infested mess," the president wrote, calling it "the worst run and most dangerous anywhere in the United States."

"No human being would want to live there," he said -- in an attack ostensibly provoked by Cummings' criticism of the harsh conditions facing would-be asylum seekers at the US-Mexico border.

US Representative Elijah Cummings speaks during Day 1 of the Democratic National Convention.

The morning diatribe ignited a storm of criticism, less than two weeks after the House of Representatives condemned Trump for "racist" comments targeting four first-term Democratic congresswomen who are ethnic minorities.

The top Democrat in Congress, Nancy Pelosi, accused Trump of a "racist" attack on a "champion... of civil rights and economic justice, a beloved leader in Baltimore, and deeply valued colleague."

"We all reject racist attacks against him," wrote Pelosi -- who was born in Baltimore and whose father served as mayor of the city.

Former vice president Joe Biden -- the Democratic frontrunner to challenge Trump in 2020 -- called out the president directly on Twitter.

"It is despicable for you to attack him and the people of Baltimore this way," Biden wrote.

A man walks by a mural painting on a rainy day in the McElderry Park area in Baltimore, Maryland.

'Disgraceful'

Democratic 2020 contender Kamala Harris, who like Booker is black, said she was "proud" to have her campaign headquarters in Cummings' district and called Trump's attack "disgraceful."

The city's Democratic Mayor Bernard "Jack" Young -- also black -- rejected Trump's rhetoric, calling it "hurtful and dangerous."

"It was a total insult," he told reporters. "We're not going to ignore anyone degrading Baltimore city and its effective leadership, no one."

Agence France-Presse