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President Trump's campaign advisers exchanged at least 18 previously undisclosed calls or emails with Russian officials during the last months of the presidential campaign, Reuters has learned from U.S. officials familiar with the situation. Six of those conversations were with Russia's ambassador to the U.S., Sergey Kislyak.

Former National Security Adviser Michael Flynn was often on the other end of communications. "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," Reuters writes, noting that was a system "both sides considered hostile to improved relations, four current U.S. officials said."

Flynn is at the heart of the government's ongoing investigation into the extent of Russia's influence over the 2016 election; in a bombshell report earlier this week, The New York Times found that President Trump apparently tried to talk his former FBI director, James Comey, out of probing Flynn's connections.

In January, the White House claimed it had no contact with any Russian officials during the campaign. People familiar with the content of the undisclosed conversations at least agree that there is no obvious evidence of collusion. But "it's rare to have that many phone calls to foreign officials, especially to a country we consider an adversary or a hostile power," said former Republican Deputy Secretary of State Richard Armitage.

The Justice Department on Wednesday appointed a special prosecutor to oversee the ongoing investigation into Russian meddling.