US President Donald Trump has lashed out against special counsel Robert Mueller's Russia investigation, branding it "an attack on our country" and exhibiting mounting concern about the year-long probe after federal authorities raided the offices of his personal lawyer.

Caught off guard and furious with the encroaching inquiry, the President showed a flare of temper watching cable news coverage of the raid yesterday, summoning lawyers Ty Cobb and Jay Sekulow to get their opinion of what was happening.

Aides and outside allies described Trump as shaken and increasingly frustrated by the development, and they said his reaction had sparked discussion about whether the raid would usher an unpredictable new phase in how the President responds to the probe.

Trump vented from the Oval Office that Mueller's investigators were "going too far" and conducting "their witch hunt" to undermine his presidency, according to three people familiar with the President's views but not allowed to discuss them publicly.

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Minutes later he publicly unleashed his sharpest invective to date against the sweeping investigation, calling the search yesterday "a disgrace". "It's an attack on our country in a true sense," he said in the Cabinet Room, flanked by the nation's top military brass, who watched the scene stone-faced. "It's an attack on what we all stand for."

Trump let loose after federal agents pierced the protective bubble around him, seizing records from the offices of long-time Trump lawyer Michael Cohen, on topics including a US$130,000 ($177,255) payment made to a porn actress who says she had sex with Trump more than a decade ago. Cohen has been an ardent defender in Trump's business, personal and political affairs for more than a decade - Cohen claims to have used a personal home equity loan to pay the adult film actress, known as Stormy Daniels - and the probe's expansion into the President's inner circle left Trump fuming.

The President didn't bat away the idea of firing Mueller, saying people have advised him to take that action: "Why don't I just fire Mueller? Well, I think it's a disgrace what's going on - we'll see what happens."

He said the raid in New York marked a "whole new level of unfairness" by Mueller and his team.

Trump called the probe a "witch hunt", as he has many times in the past, complaining that it distracts from serious issues such as consideration of a military response to the apparent use of a chemical weapon on Syrian civilians at the weekend. That was one of the intended subjects of his meeting yesterday with the Defence Secretary, the joint chiefs of staff and US combat commanders.

Instead, with reporters in the Cabinet Room and cameras rolling, Trump opened the meeting with an unprompted four-minute critique of Mueller's investigation. "I just heard that they broke into the office of one of my personal attorneys, a good man," Trump began, referring to agents who had obtained search warrants from a federal judge.

The raid on Cohen's office was conducted by the US Attorney's office in Manhattan and was based at least partly on a referral from Mueller, Cohen's lawyer, Stephen Ryan, said.

Trump blasted the Mueller team as "the most conflicted group of people I've ever seen", labelling most of them Democrats and saying the few Republicans had worked for President Barack Obama.

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"They're not looking at the other side," he complained, referencing the long investigation into former Secretary of State Hillary Clinton's use of a private email server. "They're not looking at the Hillary Clinton horrible things that she did and all of the crimes that she committed."

Mueller, a life-long Republican, and his team of attorneys have been investigating Russian interference in the 2016 election and associated misdeeds in the president's orbit since May 2017. Mueller reports to Deputy Attorney General Rod Rosenstein, who appointed the special counsel last year after the recusal of Attorney General Jeff Sessions and Trump's firing of FBI Director James Comey.

- AP