US President Donald Trump has renewed his attacks on a black US congressman from Baltimore and slammed black civil rights activist Reverend Al Sharpton, drawing a rebuke from Maryland's Republican Governor and the black former Republican Party national chairman.

Key points: Mr Trump doubled down on his criticism of congressman Elijah Cummings

Mr Trump doubled down on his criticism of congressman Elijah Cummings He also said Reverend Al Sharpton, saying he was a "conman" who "hates whites and cops"

He also said Reverend Al Sharpton, saying he was a "conman" who "hates whites and cops" Revered Sharpton said if Mr Trump thought he was a conman he would nominate him for his Cabinet

Mr Trump's weekend remarks disparaging Elijah Cummings, a House of Representatives Democrat and a longtime civil rights champion, drew charges of racism from Democrats, an accusation Mr Trump denied.

Earlier in July, the US leader attacked four racial and ethnic minority Democratic congresswomen in remarks Democrats and a number of Republicans also called racist.

Over the weekend Mr Trump called the majority-black congressional district in Baltimore that Mr Cummings represents "a disgusting, rat and rodent infested mess".

Overnight he doubled down on his criticism of Mr Cummings.

"Baltimore, under the leadership of Elijah Cummings, has the worst Crime Statistics in the Nation. 25 years of all talk, no action," he tweeted.

Mr Cummings, a veteran legislator and chairman of the House Oversight and Reform Committee, is leading a number of investigations into Mr Trump and his administration, and has criticised the President's handling of migrants from Central America.

Mr Trump also targeted the Reverend Sharpton, who has defended Mr Cummings and the congressman's district.

Reverend Sharpton later addressed the President's comments at a press conference. ( AP: Richard Drew, file )

"I have known Al for 25 years. Went to fights with him & Don King, always got along well. He "loved Trump!" He would ask me for favors often. Al is a con man, a troublemaker, always looking for a score. Just doing his thing. Must have intimidated Comcast/NBC. Hates Whites & Cops," he wrote.

Reverend Sharpton fired back in a series of tweets, in which he suggested Mr Trump had a history of shifting his stance on race.

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Speaking at an event in Baltimore later, the civil rights activist again responded to Mr Trump's comments.

"If he really thought I was a con man he'd be nominating me for his Cabinet," he said.

Many Republicans have refrained from criticising Mr Trump over his remarks. But Michael Steele, a former lieutenant governor of Maryland who served as chairman of the Republican National Committee from 2009 to 2011, spoke alongside Mr Sharpton at the press conference.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 29 seconds 29 s Former RNC chairman tells Trump to 'come on down' to Baltimore

"Mr President, your reprehensible comments are like water off a duck's back when it comes to this community. It just washes off of us," Mr Steele said, adding that Mr Trump should visit Baltimore.

"Folks want to talk to you. So just show up. Put the tweet down brother, and show up."

Maryland Governor Larry Hogan, a Republican who had considered challenging Mr Trump for the party's 2020 presidential nomination, blasted the President's tweets in an interview with WBAL radio, saying they were further fuelling division and political anger nationwide.

"The comments are just outrageous and inappropriate," Mr Hogan said.

"I think enough is enough."

'A long road to 2020'

Progressive congresswomen Ayanna Pressley, Ilhan Omar, Alexandria Ocasio-Cortez and Rashina Tlaib are known as "the Squad". ( ABC News )

Mr Trump made clear his tweets targeting Mr Cummings, as well as the four female politicians known as the "squad", were tied to his efforts to stay in the White House after the November 2020 presidential election.

"If the Democrats are going to defend the Radical Left 'Squad' and King Elijah's Baltimore Fail, it will be a long road to 2020," Mr Trump tweeted.

Like in his 2016 campaign, Mr Trump has made race a front and centre topic while Democrats are also grappling with the issue, as they seek to select a 2020 presidential nominee from nearly two dozen candidates, including several racial minorities and women.

Mr Trump's acting chief of staff Mick Mulvaney defended the President, saying the tweets against Mr Cummings were not racist.

Critics have said Mr Trump's attacks show how disconnected he is from serving as a leader of all of America, including its more racially diverse and politically liberal-leaning urban areas.

"He continues to say things about American cities all across this country," Ms Tlaib said on CNN,

"Our President has a hate agenda. He doesn't have a policy agenda."

Baltimore Mayor Bernard Young said Mr Trump should take action to help US cities.

"My message to the President is: stop tweeting and send federal help and federal resources to the city of Baltimore," he told MSNBC, adding that other cities with similar problems also needed support.

Reuters