US President Donald Trump has unleashed a scathing attack on his former personal lawyer Michael Cohen, calling him a “rat” on Twitter.

It comes after Mr Trump and his lawyer Rudy Giuliani ruled out the possibility of a presidential interview with the special counsel a week after Mr Cohen was sentenced to three years in prison for cooperating with the so-called ‘Russia probe’. 

“Remember, Michael Cohen only became a ‘Rat’ after the FBI did something which was absolutely unthinkable & unheard of until the Witch Hunt was illegally started,” Mr Trump tweeted.

The term “rat” refers to a “turncoat” or “informer” who has confessed to authorities and given law enforcement authorities inside information about a criminal organisation.

“They BROKE INTO AN ATTORNEY’S OFFICE! Why didn’t they break into the DNC to get the Server, or Crooked’s office?,” Mr Trump continued.

The initials DNC refers to the Democratic National Committee and “Crooked” is one of Trump’s favourite expressions for former Democratic presidential nominee Hillary Clinton, whom he defeated in the 2016 election.

Remember, Michael Cohen only became a “Rat” after the FBI did something which was absolutely unthinkable & unheard of until the Witch Hunt was illegally started. They BROKE INTO AN ATTORNEY’S OFFICE! Why didn’t they break into the DNC to get the Server, or Crooked’s office?

— Donald J. Trump (@realDonaldTrump) December 16, 2018

Mr Trump and Rudy Giuliani used Twitter and television interviews on Sunday to deliver a series of broadsides against special counsel Robert Mueller and federal prosecutors in New York.

Mr Giuliani said he was “disgusted” by the tactics used by Mr Mueller in his probe into Russian election interference, including in securing guilty pleas from the president’s former national security adviser Michael Flynn on a charge of lying to federal investigators.

Mr Trump, Mr Giuliani said, would not submit to an interview by Mueller’s team.

“They’re a joke,” Mr Giuliani told Fox News Sunday. “Over my dead body, but, you know, I could be dead.”

The special counsel, who is investigating possible ties between the Trump campaign and Russia, has continued to request an interview with the president.

Last month, the White House sent written answers in response to the special counsel’s questions about possible collusion.

The White House has resisted answering questions on possible obstruction of justice.

The special counsel’s investigation has spun out charges and strong-armed guilty pleas from Mr Trump underlings while keeping in suspense whether the president – “Individual-1” in Mr Mueller’s coded legalese – will end up accused of criminal behaviour himself.

This past week, his legal exposure grew as his former personal lawyer, Michael Cohen, was sentenced to three years in prison after admitting he issued hush-money payments to women who alleged sexual trysts with Trump.

Prosecutors and Cohen say he acted at the president’s direction, which Mr Trump and Mr Giuliani deny.

Mr Trump and Mr Giuliani have repeatedly tried to paint Cohen as untrustworthy, with the former New York City mayor calling him a “pathological liar”.

Mr Trump and Mr Giuliani have also accused prosecutors of intimidating the president’s associates into making false claims.

“Remember, Michael Cohen only became a ‘Rat’ after the FBI did something which was absolutely unthinkable & unheard of until the Witch Hunt was illegally started,” Trump tweeted. “They BROKE INTO AN ATTORNEY’S OFFICE!”

It was not a break-in. The FBI executed a search warrant obtained from a judge in conducting a raid in April on Cohen’s home, office and hotel room and seizing records on a variety of matters, among them a $US130,000 payment made to porn actress Stormy Daniels by Cohen.

Prosecutors have said Mr Trump directed Cohen to arrange the payments to buy the silence of Daniels and former Playboy model Karen McDougal in the run-up to the 2016 campaign.

Federal prosecutors in New York say the payments amounted to illegal campaign contributions because they were made at the height of election season to keep voters from learning of Mr Trump’s alleged infidelities.

Mr Giuliani has argued the payments were made to protect Mr Trump and Mr GiulianiTrump’s family, not to influence the election.

-with AAP