Image copyright AP Image caption Mr Manafort worked as Mr Trump's unpaid campaign chairman from March until August last year

US President Donald Trump's one-time campaign chairman secretly worked for a Russian billionaire to assist President Vladimir Putin, the Associated Press (AP) news agency reports.

Paul Manafort is said to have proposed a strategy to nullify anti-Russian opposition across former Soviet republics a decade ago.

AP says documents and interviews support its claims about Mr Manafort.

Mr Manafort has insisted that he never worked for Russian interests.

He worked as Mr Trump's unpaid campaign chairman from March until August last year, including the period during which the flamboyant New York billionaire clinched the Republican nomination.

He resigned after AP revealed that he had co-ordinated a secret Washington lobbying operation on behalf of Ukraine's ruling pro-Russian political party until 2014.

Newly obtained business records link Mr Manafort more directly to Mr Putin's interests in the region, AP says.

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It comes as Trump campaign advisers are the subject of an FBI investigation and two congressional inquiries.

Investigators are reviewing whether the Trump campaign and its associates co-ordinated with Moscow to interfere in the 2016 presidential election campaign to damage Mr Trump's opponent, Hillary Clinton, a stern critic of Mr Putin.

Mr Manafort is said to have pitched the plans to aluminium magnate Oleg Deripaska, a close ally of President Putin.

Image copyright AP Image caption Mr Deripaska is a close associate of Mr Putin

In a confidential strategy plan in 2005, AP reports, Mr Manafort proposed to influence politics, business dealings and news coverage in the US, Europe and the ex-Soviet republics to advance the interests of the Putin government.

At this time, US-Russia relations were deteriorating.

"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 is said to have written, adding that it would be offering "a great service that can refocus, both internally and externally, the policies of the Putin government".

Mr Manafort signed a $10m-a-year contract beginning in 2006, AP reports. How much work he did under this contract was unclear.

Mr Manafort and Mr Deripaska reportedly maintained a business relationship until at least 2009.

Analysis: BBC North America correspondent Anthony Zurcher

Image copyright EPA

When Donald Trump picked Paul Manafort to be his campaign chair last March, the political operative was a relatively minor player in Washington, consigned to working for deep-pocketed foreign benefactors. That those benefactors have turned out to include Russian oligarchs and Ukrainian politicians with ties to Vladimir Putin is sure to cause growing concern in the Trump White House.

Now it appears increasingly likely that Mr Manafort is one of the "individuals associated with the Trump campaign", in Director James Comey's words, at the heart of an ongoing FBI investigation.

This would explain why White House Press Secretary Sean Spicer recently downplayed Mr Manafort's connections to the Trump team, saying he "played a very limited role" in the campaign for "a very limited amount of time".

Mr Manafort could face legal consequences if the FBI concludes that he did not properly disclose his work for foreign leaders. That would at the very least prove embarrassing for Mr Trump, given the power he delegated to Mr Manafort last summer.

If it turns out that Mr Manafort's contacts with foreign interests continued during his time at the top of the Trump campaign, the situation for the White House could go from embarrassing to full-blown scandal.

In a statement to AP, Mr Manafort confirmed that he had worked for Mr Deripaska in several countries but insisted 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 in the statement.

"My work for Mr Deripaska did not involve representing Russian political interests.''

Image copyright AFP/Getty Image caption Donald Trump has been dogged by suspicion over his ties to Russian President Vladimir Putin

A spokesman for Mr Deripaska in Moscow declined to answer questions from AP.

Further allegations have been made in Ukraine about secret funds said to have been paid to Mr Manafort.

Lawmaker Serhiy Leshchenko said he had evidence that Mr Manafort had tried to hide a payment of $750,000 (£600,800) by a pro-Russian party in 2009.

Mr Manafort's spokesman said the claim was "baseless".

Mr Manafort was an adviser to Ukraine's ex-President Viktor Yanukovych, but denies receiving any cash payments.