US President Donald Trump was hit with new accusations of racism on 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.

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.

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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. "Once again you have proved yourself unfit to hold the office. A President is supposed to lift this nation up. Not tear it down."

There was similar condemnation from half a dozen White House candidates including Cory Booker, who tweeted footage of a black CNN anchor and Baltimore native who broke down on air while reacting to the attack on his hometown.

"This is painful. This is a moral, defining moment in America," wrote Booker. "Silence is toxic complicity."

'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."

The editorial board of the city's newspaper, The Baltimore Sun, took a direct swipe at Trump in a scathing column which concluded that it is "better to have a few rats than to be one".

A historic port city of 600,000 people, Baltimore presents a mixed picture, with both handsome and affluent neighborhoods and poverty-stricken districts.

It has one of the country's highest murder rates.

Cummings' district is more than 50 per cent black — and the city of Baltimore as a whole, more than 60 per cent.

Cummings himself tweeted: "Mr. President, I go home to my district daily. Each morning, I wake up, and I go and fight for my neighbors. It is my constitutional duty to conduct oversight of the Executive Branch. But it is my moral duty to fight for my constituents."

As chairman of the House Oversight Committee, Cummings — one of the most prominent African Americans in Congress — has launched investigations into Trump administration policies, including reports of poor treatment at migrant detention centers.

"Rep. Elijah Cummings has been a brutal bully, shouting and screaming at the great men & women of Border Patrol about conditions at the Southern Border, when actually his Baltimore district is FAR WORSE and more dangerous," Trump charged.

He attacked Cummings again Saturday evening after spending the day at Trump National Golf Club outside Washington.

"Elijah Cummings spends all of his time trying to hurt innocent people through 'Oversight.' He does NOTHING for his very poor, very dangerous and very badly run district!," the president tweeted, adding the hashtag #BlacksForTrump2020.

The broadside was reminiscent of Trump's recent, racially charged onslaught against four young Democratic women lawmakers, who he said should "go back" to the "crime infested" places they came from. In fact, three were born in the US and all are American citizens.

Trump denies accusations of racism — and has made a point of pushing for the release of US rapper A$AP Rocky, held in Sweden on assault charges, in an apparent effort to curb his unpopularity with black voters.

But his calculated targeting of the "Squad" — as the four are known — was widely seen as a bid to galvanize his mostly white electoral base as he gears up for next year's reelection battle.