FBI agents raided the Manhattan office of President Donald Trump's private lawyer, Michael Cohen, seizing records about Cohen's clients, including those related to a 2016 payment he made to adult-film star Stormy Daniels, who claims to have had a sexual encounter with Trump.

Among the documents seized were privileged communications between Cohen and his clients - including those with Trump, according to a person familiar with the investigators' work.

Investigators took Cohen's computer, phone and personal financial records as part of the search of his office at Rockefeller Centre, the person said.

The raid was first reported by the New York Times.

The raid is related to an investigation referred by the Special Counsel Robert Mueller to federal prosecutors in New York, according to Stephen Ryan, a lawyer for Cohen.

Ryan called the tactics "inappropriate and unnecessary," saying Cohen has "cooperated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath."

Among the records seized by investigators were "protected attorney-client communications," according to Ryan.

Dawn Dearden, spokeswoman for the US Attorney's office for the Southern District of New York, declined to comment. Peter Carr, a spokesman for the Special Counsel's office, declined to comment. A White House spokesman declined to comment.

Squire Patton Boggs, the law firm that formed a strategic alliance with Cohen last year, said in a statement that its "arrangement with Mr Cohen reached its conclusion, mutually and in accordance with the terms of the agreement.

"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."

 

CNN: A White House official said Trump had been watching TV reports of the FBI raiding Cohen's office, and that Trump knew about the raid before the news broke. https://t.co/giK9sxkDhk

— Evan Rosenfeld (@Evan_Rosenfeld) April 9, 2018

It's my understanding that government doesn't raid the offices of any lawyer lightly. The layers of bureaucratic approval that a raid of THE SITTING PRESIDENT'S LAWYER must have gone through simply boggles my mind. https://t.co/WjjZm69BsZ

— southpaw (@nycsouthpaw) April 9, 2018

Under Department of Justice regulations governing the Special Counsel's work, Mueller is required to consult with Deputy Attorney-General Rod Rosenstein if his team finds information worth investigating that does not fall under his mandate.

Rosenstein, as the acting attorney-general supervising Mueller's work, has the responsibility of deciding whether to expand Mueller's mandate to include the new topic or to refer it to a US attorney's office.

Known for his combative style and fierce loyalty to Trump, Cohen served for a decade as a top lawyer at the Trump Organisation, tangling with reporters and Trump's business competitors on behalf of the celebrity real estate mogul.

He never formally joined Trump's campaign but was in close contact with his longtime boss from his Trump Tower office throughout the 2016 race and presidential transition.

Cohen's aggressive tactics came into public view when he acknowledged he facilitated a US$130,000 payment in October 2016 to Daniels, who claims she had a sexual relationship with Trump in 2006.

Cohen left the Trump Organisation in January 2017, around the time of Trump's inauguration, and since then has served as a personal lawyer to the President.