By Veronica Stracqualursi, CNN

(CNN) -- President Donald Trump on Tuesday referred to former White House staffer Omarosa Manigault Newman, the only African-American to have served in a senior role in the White House, as a "dog."

"When you give a crazed, crying lowlife a break, and give her a job at the White House, I guess it just didn't work out. Good work by General Kelly for quickly firing that dog!" Trump tweeted Tuesday.

Referring to an African-American woman as an animal is at best a sharp departure from the language typically employed by Presidents and at worst a reference that traffics in sexual and racial imagery. Trump has long denied being racist and has dismissed a claim made by Manigault Newman that he used a racial slur on the set of "The Apprentice." He's also invoked "dog" to insult non-African-Americans -- including Mitt Romney and former White House chief strategist Steve Bannon.

As President, however, Trump has stoked racial tensions in the US, blaming "both sides" after violence sparked last year by a neo-Nazi rally in Charlottesville, Virginia. He also also privately referred some African nations as "s*hole countries" and lambasted the protests led overwhelmingly by black NFL players.

In a separate tweet Tuesday morning, Trump referred to a crash outside the UK's Houses of Parliament as a "terrorist attack" committed by "animals," though the incident was still being investigated and the identity of the driver had not been publicly released.

The President also has been accused of calling some women "fat pigs" and "dogs," though Trump has defended his treatment of women and denied multiple sexual misconduct claims against him.

Manigault Newman was fired from her job as director of communications for the Office of Public Liaison by Trump's chief of staff John Kelly last December.

Trump's attacks on Manigault Newman come as her tell-all book "Unhinged: An Insider Account of the Trump White House" is released this week, which contains several unflattering claims against the President and his staff. White House press secretary Sarah Sanders has said the book is "riddled with lies and false accusations."

Sen. Jeff Flake of Arizona condemned Trump's name-calling and said his fellow Republicans "should not be okay with it."

"This kind of language is unbecoming of a President of the United States. There is no excuse for it, and Republicans should not be okay with it," said Flake, who is not running for re-election