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.
Those discussions focused on mending U.S.-Russian economic relations strained by sanctions imposed on Moscow, cooperating in fighting Islamic State in Syria and containing a more assertive China, the sources said.
Members of the Senate and House intelligence committees have gone to the CIA and the National Security Agency to review transcripts and other documents related to contacts between Trump campaign advisers and associates and Russian officials and others with links to Putin, people with knowledge of those investigations told Reuters.
The U.S. Justice Department said on Wednesday it had appointed former FBI Director Robert Mueller as special counsel to investigate alleged Russian meddling in the U.S. presidential campaign and possible collusion between Trumps campaign and Russia. Mueller will now take charge of the FBI investigation that began last July. Trump and his aides have repeatedly denied any collusion with Russia.