Manafort and Assange say the article is false and have threatened legal action over the story.
A former aide to Donald Trump who is at the centre of the investigation into Russian meddling in the U.S. elections met Julian Assange three times in London, it has been claimed.
"I have never met Julian Assange or anyone connected to him".
Giuliani, a former federal prosecutor and New York City mayor who is representing Trump in the Mueller probe, said Manafort's lawyer told him the Guardian story is wrong.
If these reports are true, they would be extremely damaging to Manafort, Assange and many individuals within the Trump campaign, likely including the president himself.
"We are considering all legal options against the Guardian".
Manafort started cooperating in September after pleading guilty in a federal court in Washington to conspiracy against the United States - a charge that included a range of conduct from money laundering to unregistered lobbying.
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Manafort joined the Trump campaign in late March 2016, and by May had been promoted to campaign chairman.
The Guardian based its report on information from unnamed sources and an internal document from an Ecuadorian intelligence agency.
Manafort, meanwhile, has violated the plea agreement he negotiated with prosecutors after that conviction in August, according to court documents filed on Monday. The next month they hacked into the Democratic Congressional Campaign Committee and the Democratic National Committee.
"I don't know about it and if I don't know about it, he doesn't know it", Giuliani told the Daily News.
So far the parties involved have either strongly denied the report or, like press secretary Sarah Sanders, demurred from commenting upon it one way or another.
Manafort's lawyers declined answering The Guardian's questions about the alleged "secret talks" with Assange.
Special Counsel Robert Mueller started his investigation nearly 1.5 years ago but has still failed to present evidence proving alleged collusion between Donald Trump's presidential campaign and Moscow. However, the timing is suspicious as his last visit, in March 2016, came just months before WikiLeaks released of a stash of Democratic emails stolen by Russian intelligence officers.
Still, many people disagreed with claims that Mueller would be able to prove Trump's alleged misdeeds, noting that the 1.5 year long investigation had failed to find any proof of his alleged ties to Moscow.