Updated 7.45pm with more testimony by Michael Cohen -

Michael Cohen, Donald Trump's longtime lawyer and fixer, assailed his former former boss as a "racist" and a  "conman" in explosive testimony before the US Congress Wednesday.

Addressing the House Oversight and Reform Committee, Cohen - who has been sentenced to jail for crimes related in part to his work for Trump - expressed regret for his past loyalty.

"I am ashamed that I chose to take part in concealing Mr. Trump's illicit acts rather than listening to my own conscience," Cohen said.

"He is a racist. He is a conman. He is a cheat."

Cohen said he was presenting "irrefutable" evidence of Trump's wrongdoing including a cheque reimbursing "hush money" paid to two women shortly before the 2016 election. 

He also said Trump directed negotiations for a Trump Tower in Moscow through the 2016 election campaign even while denying any business ties with the Russians.

A copy of a check paid to Michael Cohen by President Trump is displayed as Michael Cohen, former attorney and fixer for President Donald Trump testifies before the House Oversight Committee on Capitol Hill.

Cohen also said the US president had no medical reason for his military deferment during the Vietnam War, telling the lawyer: "You think I'm stupid? I wasn't going to Vietnam." 

Trump, who is in Hanoi for a summit with North Korean leader Kim Jong Un, received a medical deferment from serving in the Vietnam War in 1968 because of bone spurs in his heels.

But Cohen declared Mr Trump told his former attorney and right hand man for more than a decade that there was no real medical excuse behind his deferral.

"Mr. Trump tasked me to handle the negative press surrounding his medical deferment from the Vietnam draft," Cohen said. 

"Mr Trump claimed it was because of a bone spur, but when I asked for medical records, he gave me none and said there was no surgery... He told me not to answer the specific questions by reporters but rather offer simply the fact that he received a medical deferment."

The deferment was one of five Mr. Trump received during Vietnam. The others were for education.

Cohen said Trump then added: "You think I'm stupid? I wasn't going to Vietnam."

The New York Times reported in December that the family of the podiatrist who wrote the letter attesting to Trump's medical condition had done so as a favour to the future president's father, a New York real estate developer who owned the building where the doctor practised.

'Trump is becoming an autocrat'

Cohen lashed out at Republicans for blind support of Donald Trump, saying Trump foments incivility, habitually lies and abuses his power to the point of becoming an autocrat.

"I can only warn people, the more people that follow Mr Trump as I did blindly are going to suffer the same consequences that I'm suffering," Cohen said.

"Look at what's happened to me? I had a wonderful life. I have a beautiful wife. I have two amazing children. I achieved financial success by the age of 39. I didn't go to work for Mr Trump because I had to. I went to work for him because I wanted to, and I've lost it all."

Cohen  said the real estate tycoon is abusing his power in the White House, including making threats against Cohen's family on Twitter, where the president has tens of millions of followers.

"He's got over 60 million people," Cohen told the committee."When he goes on Twitter and he starts bringing in my in-laws, my parents, my wife, what does he think is going to happen? He's sending out the same message that he can do whatever he wants: this is his country," Cohen continued.

"He's becoming an autocrat, and hopefully something bad will happen to me or my children or my wife so that i will not be here and testify."

"You don't know him. I do. I've sat next to this man for 10 years, and I watched his back," Cohen added.

Republicans in the hearing sought to discredit Cohen as not credible, pointing to his guilty plea last year to charges of lying to Congress, illegal hush-money payments to a former lover of Trump, and tax evasion.

But Cohen stiffly rebutted their charges, saying he had broken the law but is now coming clean about a president for whom he is now "ashamed" of having worked.He countered that Republicans were -- like he had before -- protecting Trump at all costs by trying to ignore the president's behaviour.

From committee Republicans, Cohen chided, "not one question so far since I'm here has been asked about Mr Trump."

Cohen, 52, worked closely with Trump for more than 12 years, rising to vice president of the Trump Organization, where he was the billionaire property magnate's behind-the-scenes "fixer."

Cohen has been sentenced to three years in jail for crimes related in part to his work for Trump. He testified to the Senate in a closed-doors session on Tuesday and appeared on Wednesday in an open session before House of Representatives Oversight Committee.