US President Donald Trump has hit out at a prominent African-American critic, calling him a “brutal bully” who should concentrate on cleaning up his “disgusting, rat and rodent infested” Baltimore district rather than criticising the work of US immigration officers on the Mexican border.

Trump’s target in a series of early morning tweets was US Representative Elijah Cummings, Democratic chairman of the House of Representatives Oversight Committee, who has called Trump a racist and sharply criticised his immigration policies.

Democratic House Speaker Nancy Pelosi responded to Trump by calling Cummings a champion for civil rights and economic justice. She added in a tweet:  “We all reject racist attacks against him and support his steadfast leadership.”

The president’s assault on a senior African-American leader follows his attack just two weeks ago on four Democratic congresswomen of colour in which he said they should “go back” to the countries they came from.

Only one of the congresswomen was not born in the US and all are American citizens.

On Thursday, Cummings’ committee voted 23-16 along party lines to allow Cummings to issue subpoenas to White House officials, including Trump’s daughter Ivanka Trump and son-in-law Jared Kushner, deepening a probe into potential violations of government record-keeping laws.

Later on Saturday, Trump signaled his anger with Cummings’ committee efforts, writing in another tweet that the Democrat “spends all of his time trying to hurt innocent people through ‘Oversight.'”

“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. His district is considered the Worst in the USA……,” Trump tweeted.

“(T)he Border is clean, efficient & well run, just very crowded. 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 said.

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! Take a look…. #BlacksForTrump2020 https://t.co/seNVESZUht

— Donald J. Trump (@realDonaldTrump) July 27, 2019

In another tweet, Trump questioned why so much money was spent on Cummings’ district, which covers the downtown area of the city of Baltimore, about 64km northeast of Washington, “when it is considered the worst run and most dangerous anywhere in the United States”.

“No human being would want to live there. Where is all this money going? How much is stolen? Investigate this corrupt mess immediately,” Trump wrote.

Last week, Cummings slammed Trump’s acting Homeland Security Secretary Kevin McAleenan over conditions of US-Mexico border facilities that house migrants.

-AAP