Current and former U.S. officials familiar with the exchanges told Reuters, Censor.NET reports.
The previously undisclosed interactions form part of the record now being reviewed by FBI and congressional investigators probing Russian interference in the U.S. presidential election and contacts between Trump's campaign and Russia.
Six of the previously undisclosed contacts described to Reuters were phone calls between Sergei Kislyak, Russia's ambassador to the United States, and Trump advisers, including Flynn, Trump's first national security adviser, three current and former officials said.
Conversations between Flynn and Kislyak accelerated after the Nov. 8 vote as the two discussed establishing a back channel for communication between Trump and Russian President Vladimir Putin that could bypass the U.S. national security bureaucracy, which both sides considered hostile to improved relations, four current U.S. officials said.
In January, the Trump White House initially denied any contacts with Russian officials during the 2016 campaign. The White House and advisers to the campaign have since confirmed four meetings between Kislyak and Trump advisers during that time.
The people who described the contacts to Reuters said they had seen no evidence of wrongdoing or collusion between the campaign and Russia in the communications reviewed so far. But the disclosure could increase the pressure on Trump and his aides to provide the FBI and Congress with a full account of interactions with Russian officials and others with links to the Kremlin during and immediately after the 2016 election.
The White House did not respond to requests for comment. Flynn's lawyer declined to comment. In Moscow, a Russian foreign ministry official declined to comment on the contacts and referred Reuters to the Trump administration.
Separately, a spokesman for the Russian embassy in Washington said: "We do not comment on our daily contacts with the local interlocutors."
The 18 calls and electronic messages took place between April and November 2016 as hackers engaged in what U.S. intelligence concluded in January was part of a Kremlin campaign to discredit the vote and influence the outcome of the election in favor of Trump over his Democratic challenger, former secretary of state Hillary Clinton.
In addition to the six phone calls involving Kislyak, the communications described to Reuters involved another 12 calls, emails or text messages between Russian officials or people considered to be close to Putin and Trump campaign advisers.
One of those contacts was by Viktor Medvedchuk, a Ukrainian oligarch and politician, according to one person with detailed knowledge of the exchange and two others familiar with the issue.
It was not clear with whom Medvedchuk was in contact within the Trump campaign but the themes included U.S.-Russia cooperation, the sources said. Putin is godfather to Medvedchuk's daughter.
Medvedchuk denied having any contact with anyone in the Trump campaign.
"I am not acquainted with any of Donald Trump's close associates, therefore no such conversation could have taken place," he said in an email to Reuters.
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