US President Donald Trump did not adhere to warnings from his advisers when he congratulated Vladimir Putin on his re-election, US media report.
A note to the president for his phone call with his Russian counterpart reportedly read: "DO NOT CONGRATULATE".
Mr Putin's controversial win came as his opponent was barred from the ballot, and some world leaders have been hesitant to congratulate him.
Mr Trump described the conversation with the Kremlin as a "very good call".
He defended his call on Twitter on Wednesday afternoon.
The White House said in a statement that it's a "fireable offence and likely illegal" to have leaked the president's briefing papers, according to US media.
US Republican Senator John McCain criticised Mr Trump over the call.
"An American president does not lead the free world by congratulating dictators on winning sham elections. And by doing so with Vladimir Putin, President Trump insulted every Russian citizen who was denied the right to vote in a free and fair election to determine their country's future, including the countless Russian patriots who have risked so much to protest and resist Putin's regime," Mr McCain said in a statement.