Donald Trump has blasted the investigation led by special counsel Robert Mueller as a "disgrace" and a "witch-hunt" after FBI agents raided the New York offices of the president's long time personal lawyer Michael Cohen.

Agents seized files relating to Mr Cohen's work, which included documents regarding a $130,000 payment before the 2016 election to an adult film actress who says she had sex with Mr Trump a decade earlier.

Mr Cohen's own attorney Stephen Ryan said agents were working in part on the request of Mr Mueller, the independent prosecutor investigating links between Russia and the Trump campaign.

At the White House, Mr Trump angrily denounced the raid on "good man" Mr Cohen.

"It's a disgraceful situation. It's a total witch-hunt, I have been saying it for a long time," he told reporters.

"It's an attack on our country in a true sense. It's an attack on what we all stand for."

The raid came as Mr Mueller's probe intensifies its focus on Mr Trump's inner circle.

Mr Mueller's team is examining possible collusion between Mr Trump's campaign and Russia, as well as allegations of corrupt behaviour by Trump campaign lieutenants and of White House efforts to obstruct the investigation.

So far, 19 people have been indicted, including the chairman of Mr Trump's campaign and his former national security advisor.

President Trump accused Mr Mueller of overseeing a biased investigation staffed by Democrats, "a pure and simple witch-hunt".

But the top Democrat on the House Intelligence Committee, Adam Schiff, said it was a Trump-appointed prosecutor who had sought the search warrant, which was in turn approved by a judge.

"That's no witch-hunt. That's the rule of law," Mr Schiff tweeted.

After having turned over what he called a million pages of documents, Mr Trump said the investigation is "now getting ridiculous. They found no collusion whatsoever with Russia".

"This is the most biased group of people, these people have the biggest conflicts of interest I've ever seen. Democrats all, or just about all."

Asked if he wants to fire Mueller, Mr Trump replied: "Many people have said: 'You should fire him'. Again they found nothing and in finding nothing that's a big statement. So we'll see what happens."

A veteran prosecutor and a former FBI director, Mr Mueller is seeking to interview Mr Trump, but has not said the president is a target in the investigation.

The raid on Mr Cohen though takes the probe close to the Oval Office.

Mr Cohen has been Mr Trump's personal lawyer and confidant for years, helping him on property and personal matters, as well as informally advising him since he became president.

He is also known as a rough-and-tumble "fixer" for Mr Trump, dedicated to protecting his boss's interests.

The New York Times, citing a source, said FBI agents seized emails, tax documents and business records.