The chairman of President Donald Trump's 2016 election campaign secretly shared campaign information with a Russian intelligence officer, posing a "grave" espionage threat to the United States, a US Senate report said Tuesday.

Before and during his nearly six months on the Trump campaign, Paul Manafort, a veteran Republican political consultant, directly and indirectly communicated with  Konstantin Kilimnik, identified as a Russian intelligence officer, and Oleg Deripaska, a Russian oligarch close to Vladimir Putin, the Senate Intelligence Committee said. "On numerous occasions, Manafort sought to secretly share internal campaign information with Kilimnik," including polling and strategy details, it said.

While the reason for the sharing was not clear, the report said, it noted that it took place just as Russian GRU intelligence and a government-linked social media operation actively sought to tilt the election toward Trump.

Manafort "was secretly communicating with a Russian intelligence officer ... while the Russian intelligence operation to assist Trump was ongoing," the report said.

"Taken as a whole, Manafort's high-level access and willingness to share information with individuals closely associated with Russian intelligence services, particularly Kilimnik and associates of Oleg Deripaska, represented a grave counterintelligence threat."

The Senate report, the final product of a three-year investigation into Russian election meddling in 2016, described numerous incidents in which the Trump campaign actively sought the help of Moscow and WikiLeaks to damage the campaign of Trump rival Hillary Clinton.

The report came as US intelligence has warned of active Russian attempts to interfere in the current presidential race in Trump's favor.

'Complexity of the threat' 

Much of the Senate report repeats the conclusions of the Justice Department special counsel investigation of Robert Mueller.

Mueller outlined numerous contacts between the campaign and Russia, but, because his focus was on whether criminal acts took place, he simply concluded that there were no grounds for charges.

"The special counsel's inability to 'establish' a criminal conspiracy between the Trump campaign and Russia does not convey the breadth and complexity of the threat presented by their actions," the Senate report says.

Trump has repeatedly denied colluding with or reaping support from Russia, calling investigations a "witch hunt."

Manafort, who was dropped from the campaign in August 2016, nearly three months before the election, had known Kilimnik and Deripaska for years from his work with them in Ukraine, where he was a political consultant, the report notes.

But it said it had no explanation for his secrecy in transmitting to them internal campaign information.

Manafort was investigated by Mueller and eventually convicted on multiple charges of tax and bank fraud and money laundering related to his Ukraine business, but unrelated to the Trump campaign.

Sentenced last year to seven and a half years in jail, the 71-year-old was released in May to home confinement to protect him from the coronavirus threat.

Trump has repeatedly suggested he could pardon Manafort.