A top US lawyer says President Donald Trump is sailing perilously close towards impeachment.

Sacked FBI director James Comey has appeared before a Senate Intelligence Committee.

He said the Trump administration defamed him and told lies about the FBI.

"The adminstration then chose to defame me, and more importantly the FBI, by saying that the organisation was in disarray, that it was poorly led, that the workforce had lost confidence in its leader. Those were lies, plain and simple," Comey said.

The Senate Intelligence Committee is looking into whether Russia meddled with last year's election, and had ties with Trump's campaign.

New York attorney Scott Horton told Rachel Smalley Trump has effectively been accused of bribery, by threatening to fire Comey if he didn't carry out the President's wishes.

"His continuation as FBI Director was being dangled before him in exchange for some confessions and they were very specific asks to do with the Russian investigation."

LISTEN ABOVE AS NEW YORK ATTORNEY SCOTT HORTON SPEAKS TO RACHEL SMALLEY

Comey was asked why he kept detailed notes of his meetings with President Trump.

"I was honestly concerned he might lie about the nature of our meeting and so I thought it really important to document. That combination of things I'd never experienced before but it led me to believe I've got to write it down in a very detailed way."

Comey wouldn't say if he thinks Trump sought to obstruct justice, by asking him to drop an investigation into former national security advisor Michael Flynn.

But he said he perceived it as an order.

The White House says Donald Trump's lawyer will give a statement after the lengthy testimony.

But it's saying already - that Mr Trump is not a liar.

Trump says he and his supporters are under siege, but will come out stronger than ever.

The president didn't make specific reference to Comey, but referred to being under siege.

Mr Trump told an audience he won't back down from doing what is right.