The US President Donald Trump blasted the investigation led by special counsel Robert Mueller as a disgrace and a witch hunt Monday after FBI agents raided the New York offices of the president's longtime personal lawyer Michael Cohen.

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

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

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

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

Trump accused 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.