President Trump is reportedly furious over a leak to The Washington Post about how he overrode his national security advisers' warnings not to congratulate Russian President Vladimir Putin on his recent election victory.

The report said Trump's briefing materials had included the warning, "DO NOT CONGRATULATE" in all caps, but that Trump did not heed it.

He also didn't follow other pointers that said to condemn Russia for a recent nerve agent attack in the UK and Russia's election interference.

The leak is said to have angered Trump, who believes some within his national security apparatus are secretly working to undermine his presidency.

It's unclear if Trump saw all the briefing notes.

President Donald Trump is apparently fuming over a leak to the media about how he overrode advice from his national security advisers recommending against congratulating Russian President Vladimir Putin on his election victory during a phone call on Tuesday.

CNN reported Wednesday that Trump asked several allies and aides on Tuesday night who was responsible for the leak, adding that it could only have come from a select few individuals with access to sensitive national security information. The New York Times' Maggie Haberman cited a White House official who said leaking such information "is a fireable offense and likely illegal."

The leak resulted in a bombshell Washington Post report, which said advisers included a warning in Trump's briefing materials, in all caps: "DO NOT CONGRATULATE" Putin on winning the Russian election last week.

They also instructed him to condemn Putin for the chemical attack on a former Russian spy in the UK, which both the US and UK said was ordered by the Kremlin.

It's unclear whether Trump viewed all of the briefing materials.

Trump apparently did not follow either suggestion. Nor did he press Putin on Russia's interference in the 2016 election.

One source told CNN on Wednesday that the leak rekindled Trump's belief that some within his administration, particularly in the national security apparatus, are working to undermine his presidency.

Trump's decision to congratulate Putin on his victory prompted immediate criticism from members of Congress more hawkish toward Russia, like Arizona Republican Sen. John McCain.

On Sunday, Putin won more than 75% of the vote in Russia's presidential election, securing him at least another six years in power. There have been allegations of ballot-stuffing during the election, and election monitors were harassed and intimidated in the run-up to the vote.

'The Russians may have something on him'

Meanwhile, critics laid into Trump earlier this month for his initially lukewarm response to the nerve-agent attack on former Russian military intelligence officer Sergei Skripal and his daughter Yulia, both of whom are currently on life support.

Trump's decision Tuesday to ignore his advisers' guidance and congratulate Putin adds another layer to an emerging portrait in which the US president has demonstrated a reluctance to confront the Russian leader amid an ongoing investigation into whether the Trump campaign colluded with Moscow to tilt the 2016 election in his favor.

Former CIA Director John Brennan told MSNBC Wednesday morning that he was baffled by Trump's unwillingness to push back against Putin.

"I think he's afraid of the president of Russia," Brennan said. "I think one can speculate as to why, that the Russians may have something on him personally that they could always roll out and make his life more difficult."

Brennan added that while it was important for the US and Russia to see improved relations, "the fact that he's had this fawning attitude toward Mr. Putin, has not said anything negative about him, continues to say to me that he has something to fear."

A White House official told Axios on Wednesday, "This is the way Trump is. If he's doing business with you or working with you in some way, he's going to congratulate you."

The official added that the notion that Trump is soft on Russia "is crap." At the same time, they said, "he doesn't want his personal relationship [with Putin] to be acrimonious," and that Trump needs to connect to Putin on a "personal, leader-to-leader level" to rebuild US-Russia relations.