Senior officials who have reviewed the phone call between the two men told The Washington Post they thought Mr Flynn’s statements to Mr Kislyak were inappropriate, if not illegal, because he suggested that the Kremlin could expect a reprieve from the sanctions.

Mr Flynn, now being investigated as part of Robert Mueller’s investigation into Russia’s involvement in the US election, has always insisted he did not discuss sanctions.

And on Saturday Mr Kislyak, speaking for the first time about his conversations with the former army lieutenant general, denied anything improper had taken place.

"We only spoke about the most simple things, but the communication was completely correct, calm, absolutely transparent,” said Mr Kislyak, in a panel discussion on Russian television.