The special counsel investigating whether Russia tried to sway the 2016 US election has taken over FBI inquiries into a former British spy's dossier of allegations of Russian financial and personal ties to President Donald Trump, according to sources familiar with the probe.

Key points: The dossier contains highly salacious-yet-unverified material about Mr Trump

The dossier contains highly salacious-yet-unverified material about Mr Trump Sources familiar with Mr Mueller's probe now say aspects of it have become of interest

Sources familiar with Mr Mueller's probe now say aspects of it have become of interest The Senate panel says it is aiming to conclude its probe by the end of this year

A scandalous report compiled by former MI6 officer Christopher Steele that surfaced last year identified Russian businessmen and others who US intelligence analysts have since concluded are Russian intelligence officers or working on behalf of the Russian Government.

Keeping with protocol, the spokesman for special counsel Robert Mueller has declined to comment, but three sources with knowledge of Mr Mueller's probe said his investigators had assumed control of multiple FBI inquiries related to the dossier that Russia interfered to help Mr Trump defeat Hillary Clinton.

Russia has repeatedly denied any meddling in the election.

Mr Trump, who has called allegations of campaign collusion with Moscow a hoax, has faced questions about the matter since he took office in January.

Although several news organisations were briefed on Mr Steele's dossier before the election in November, most decided not to report on the material at the time because it contained inflammatory and salacious content that could not be independently verified.

However, in a report published in January, four US intelligence agencies said they had taken the dossier's allegations seriously.

Mr Trump was reportedly told by former FBI director James Comey that Mr Steele's report contained salacious material about the businessman-turned-president, which he has maintained is "fake news" and part of a political "witch hunt".

But aspects of interest to Mr Mueller are reportedly not necessarily related to the salacious content, but parts relating to personal and financial ties of Mr Trump's campaign associates to the Kremlin.

The information on Mr Trump collected by Mr Steele, who officials say was one of MI6's most respected Russia hands, was laid out last year in political "opposition research" initially financed by supporters of one of Mr Trump's Republican primary election opponents.

After Mr Trump won the Republican nomination in July, backers of Mrs Clinton picked up the support of Mr Steele's work.

Senate committee aiming to conclude probe by 2018

On Wednesday, the Senate Intelligence Committee's chairman Richard Burr told reporters the issue of whether Mr Trump's campaign colluded with Russia remains an open question.

Senate panel chairman Richard Burr gave an update on the ongoing investigation on Wednesday. ( Reuters: Aaron P Bernstein )

"We have not come to any determination on collusion," Mr Burr said.

Mr Burr said the Senate panel had made several attempts to contact Mr Steele and to meet him and "those offers have gone unaccepted".

"The committee cannot really decide the credibility of the dossier without understanding things like who paid for it, who are your sources and sub-sources," Mr Burr said.

Mr Burr said the panel wanted to finish its investigation by the end of the year.

Meanwhile, two officials familiar with the investigations said both Mr Mueller's team and the Senate panel are also seeking any evidence that former Trump campaign manager Paul Manafort or others who had financial dealings with Russia might have helped Kremlin intelligence agencies target email hacking and social media postings benefitting Mr Trump.

Reuters/ABC