Donald Trump's former campaign chairman Paul Manafort secretly worked for a Russian billionaire to advance the interests of Russian President Vladimir Putin, it has been revealed.
Mr Manafort wrote a 2005 strategy plan that he said "can greatly benefit the Putin Government", the Associated Press reported.
At the time, US-Russia relations under Republican president George W Bush were growing worse.
Mr Manafort's arrangement was with Russian aluminium magnate Oleg Deripaska, a close Putin ally.
Mr Manafort signed a 10 million dollar (£8m) annual contract beginning in 2006 and maintained a business relationship until at least 2009.
The work was described in interviews with people familiar with it and confidential business records obtained by the AP.
Mr Manafort confirmed that he worked for Mr Deripaska but said the work was being unfairly cast as inappropriate.
Mr Manafort proposed in a confidential strategy plan as early as June 2005 that he would influence politics, business dealings and news coverage inside the United States, Europe and the former Soviet republics to benefit the Putin government.
"We are now of the belief that this model can greatly benefit the Putin Government if employed at the correct levels with the appropriate commitment to success," Mr Manafort wrote in the 2005 memo to Mr Deripaska.
He said the model "will be offering a great service that can re-focus, both internally and externally, the policies of the Putin government".
The plans were laid out in documents obtained by the AP that included strategy memoranda and records showing international wire transfers for millions of dollars. How much work Mr Manafort performed under the contract was unclear.
The disclosure comes as Trump campaign advisers are the subject of an FBI probe and two congressional investigations.
Investigators are reviewing whether the Trump campaign and its associates coordinated with Moscow to meddle in the 2016 campaign.
Mr Manafort has dismissed the investigations as politically motivated and misguided, and said he never worked for Russian interests. The documents obtained by AP show Mr Manafort's ties to Russia were closer than previously revealed.
In a statement, Mr Manafort confirmed that he worked for Mr Deripaska in various countries but said the work was being unfairly cast as "inappropriate or nefarious" as part of a "smear campaign".
"I worked with Oleg Deripaska almost a decade ago representing him on business and personal matters in countries where he had investments," Mr Manafort said. "My work for Mr Deripaska did not involve representing Russian political interests."
Mr Deripaska became one of Russia's wealthiest men under Mr Putin, buying assets abroad in ways widely perceived to benefit the Kremlin's interests.
US diplomatic cables from 2006 described Mr Deripaska as "among the 2-3 oligarchs Putin turns to on a regular basis" and "a more-or-less permanent fixture on Putin's trips abroad".
In response to questions about Mr Manafort's consulting firm, a spokesman for Mr Deripaska in 2008 - at least three years after they began working together - said Mr Deripaska had never hired the firm. Another Deripaska spokesman in Moscow last week declined to answer AP's questions.
Mr Manafort worked as Mr Trump's unpaid campaign chairman last year from March until August.
Mr Trump asked him to resign after AP revealed that Mr Manafort had orchestrated a covert Washington lobbying operation until 2014 on behalf of Ukraine's ruling pro-Russian political party .
FBI director James Comey, in confirming to Congress the federal intelligence investigation this week, declined to say whether Mr Manafort was a target. His name was mentioned 28 times during the hearing of the House Intelligence Committee, mostly about his work in Ukraine but no-one mentioned Mr Deripaska.
White House spokesman Sean Spicer said on Monday that Mr Manafort "played a very limited role for a very limited amount of time" in the campaign, even though as Mr Trump's presidential campaign chairman he led it during the crucial run-up to the Republican National Convention.
Mr Manafort and his associates remain in Mr Trump's orbit. Mr Manafort told a colleague this year that he continues to speak with Mr Trump by telephone.
His former business partner in eastern Europe, Rick Gates, has been seen inside the White House on a number of occasions. Mr Gates has since helped plan Mr Trump's inauguration and now runs a non-profit organisation, America First Policies, to back the White House agenda.