WASHINGTON (BLOOMBERG) - US President Donald Trump angrily denounced an FBI raid on his private lawyer's offices on Monday (April 9), describing it as a "disgrace" and "whole new level of unfairness".

Railing against what he described as a "witch hunt" and "an attack on our country", Mr Trump pilloried his own attorney general and noted that "many people" were advising him to fire Special Counsel Robert Mueller.

"They found no collusion whatsoever with Russia," a visibly livid Trump said, hours after officials seized documents and other evidence from his attorney Michael Cohen.

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

Mr Trump said it was a "terrible mistake" for Attorney General Jeff Sessions to recuse himself from Mr Mueller's investigation of Russian meddling in the 2016 election.

The referral to the US attorney in the Southern District of New York was approved by Deputy Attorney General Rod Rosenstein, according to a person familiar with the situation.

The President said agents "broke into" Mr Cohen's office in midtown Manhattan, even though a federal judge had to approve the search warrants used by the FBI.

Mr Cohen has been a key figure in several legal issues, including the investigations into Russian election meddling and a payment to an adult film actress.

Mr Cohen, a longtime lawyer for the Trump Organisation, paid US$130,000 (S$170,466.40) to Ms Stephanie Clifford, an adult film actress known as Stormy Daniels. She has said Mr Cohen paid her to keep quiet about having sex with Trump in 2006, the year after he married Melania Trump. Mr Cohen said he made the payment without Mr Trump's knowledge, using his own money.

Mr Cohen's attorney Stephen Ryan said agents on Monday seized documents using several search warrants after a referral to prosecutors in the Southern District of New York based in part on a referral by Special Counsel Robert Mueller.

Agents raided the office, home and a Manhattan hotel room of Mr Cohen, the Wall Street Journal reported. Agents were still at the site of Mr Cohen's office at Rockefeller Centre in midtown Manhattan late on Monday afternoon, a worker there said. He had an office there in the space of Squire Patton Boggs, but they're ended their agreement, the firm said in a statement.

"We have been in contact with federal authorities regarding their execution of a warrant relating to Mr Cohen," the firm said. "These activities do not relate to the firm and we are in full cooperation."

Mr Ryan said the use of search warrants was "completely inappropriate and unnecessary" because it resulted in the seizure of material protected by attorney-client privilege.

"Those government tactics are also wrong because Mr Cohen has cooperated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath," according to Mr Ryan.

The raid signals that the FBI and federal prosecutors in New York are conducting a criminal investigation involving Mr Cohen, although it's not immediately clear whether he is a focus of that probe.

Mr Trump, who has denied having an affair with Ms Daniels, said last week that he was unaware of the payment. Mr Cohen and Ms Daniels, whose real name is Stephanie Clifford, have been involved in a legal battle over the agreement.

FBI spokeswoman Amy Thoreson and Justice Department spokeswoman Sarah Isgur Flores declined to comment. The FBI raid was reported earlier by the New York Times.

Mr Mueller brought information involving Mr Cohen to Deputy Attorney General Rod Rosenstein, who decided that the matter should be handled by the US attorney for the Southern District of New York rather than by Mr Mueller's team, according to a person familiar with the matter.

Under Justice Department regulations, Mr Mueller is required to consult with Mr Rosenstein about how to handle evidence and matters that fall outside his jurisdiction and authority.

Mr Mueller is investigating Russian meddling in the 2016 US presidential campaign, whether anyone close to Mr Trump colluded in it and whether Mr Trump sought to obstruct justice.

Mr Michael Avenatti, a lawyer for Ms Daniels, said in an e-mail that Mr Cohen "has been placed in the crosshairs by Mr Trump. He has been set up to take the fall."

Mr Cohen has denied any collusion with Russia. He was questioned by congressional investigators last year over his connection to Russia, including a deal for a Trump Tower in Moscow.

Mr Cohen has told congressional investigators that the Trump Organisation weighed a proposal to build a hotel and condominium tower in Moscow. Mr Cohen said the company signed a non-binding letter of intent in October 2015 with Moscow-based IC Expert Investment Company.

The Trump Organisation solicited building designs from architects and engaged in preliminary financing discussions. But the project ultimately fizzled, and Mr Trump wasn't involved in the decision to abandon it, Mr Cohen said.

The discussions, which continued into the 2016 campaign, were about a "real estate deal and nothing more", Mr Cohen said.

The raid suggests Mr Mueller may have documents that call into question the credibility of Mr Trump and Mr Cohen, even if the materials have nothing to do with Russian meddling, said Mr Jeffrey Cohen, a New York family law attorney who's not involved in the case and isn't related to Mr Michael Cohen.

"Someone had to have fed Mueller information to make him believe that either Trump and/or Cohen were not being honest in the statements they've made publicly," Mr Cohen said. "It could only have to do with lying."