Democratic leaders in Congress on Wednesday angrily walked out of a White House meeting with President Donald Trump, with House Speaker Nancy Pelosi claiming the president had a "meltdown", NBC News reported.

"What we witnessed on the part of the president was a meltdown. Sad to say," Pelosi told reporters outside the White House with House Majority Leader Steny Hoyer and Senate Minority Leader Chuck Schumer.

The President started the meeting with a lengthy bombastic monologue, said a senior Democratic aide. He bragged about the "nasty" letter he sent to Turkish President Recep Tayyip Erdogan over the Turkish leader's decision to invade northern Syria, the aide said.

The meeting quickly devolved into a series of contentious exchanges centering on the president's decision earlier this month to pull troops from Syria, according to NBC News.

The White House called the meeting to discuss the President’s decision and the deescalating situation on the ground. Before the meeting, the House overwhelmingly voted in favor of a resolution rebuking Trump’s decision to pull troops out in a 354-60 vote.

"I think that vote, the size of the vote — more than 2 to 1 of the Republicans voted to oppose what the president did — it probably got to the president, because he was shaken up by it," Pelosi was quoted as having said. "That's why we couldn't continue in the meeting because he was just not relating to the reality of it."

At one point during the meeting, Schumer brought up former Defense Secretary James Mattis, who told NBC's "Meet the Press" on Sunday that Trump's decision to pull troops from Syria could lead to ISIS's resurgence.

According to multiple aides, Trump called Mattis, "the world's most overrated general."

"You know why?," Trump said, according to one aide. "He wasn't tough enough. I captured ISIS. Mattis said it would take two years. I captured them in one month."

In another heated encounter, according to the report, Pelosi challenged Trump on Russia's involvement in Syria, saying "All roads with you lead to Putin."

The Democratic leaders said that the moment that prompted them to abruptly leave was when Trump called Pelosi "a third-rate politician" to her face.

Pelosi and Hoyer then stood up and left the meeting, the aide said.

As they left said, Trump shot back, "Goodbye, we'll see you at the polls." Schumer followed shortly thereafter, according to NBC News.

"He was insulting, particularly to the speaker," Schumer told reporters later on Wednesday. "She kept her cool completely. But he called her a third-rate politician. He said that there are communists involved and you guys might like that. I mean, this was not a dialogue. It was sort of a diatribe — a nasty diatribe not focused on the facts, particularly the fact of how to curtail ISIS, a terrorist organization that aims to hurt the United States in our homeland."

Hoyer echoed those remarks, saying that the meeting "deteriorated into a diatribe" and that they were "deeply offended" by the way Trump treated Pelosi. He said that after serving in Congress over the course of six presidential administrations, he has "never" seen a president "treat so disrespectfully a co-equal branch of the government."

White House Press Secretary Stephanie Grisham denied the Democrats’ characterization of the meeting.

"The President was measured, factual and decisive, while Speaker Pelosi’s decision to walk out was baffling, but not surprising. She had no intention of listening or contributing to an important meeting on national security issues. While Democratic leadership chose to storm out and get in front of the cameras to whine, everyone else in the meeting chose to stay in the room and work on behalf of this country," said Grisham.

Trump tweeted a series of photos from the meeting, including one that he labeled "Nervous Nancy's unhinged meltdown."

He later wrote, “Nancy Pelosi needs help fast! There is either something wrong with her “upstairs,” or she just plain doesn’t like our great Country. She had a total meltdown in the White House today. It was very sad to watch. Pray for her, she is a very sick person!”

Republicans, who stayed behind in the meeting, spoke to reporters afterward. House Minority Leader Kevin McCarthy (R-CA) said that Pelosi stormed out of the room, calling it “unbecoming” of the speaker.

"When there is a time of crisis, leaders should stay" whether they like what they’re hearing or not, he added.

The meeting was held in the wake of tensions between Trump and the Democrats over impeachment proceedings against the president.

Pelosi announced last month that she would be launching formal impeachment proceedings against Trump over allegations that he threatened to withhold aid to Ukraine unless its government investigated actions by his Democratic rival, former Vice President Joe Biden.

On Tuesday, Pelosi said at a press conference that she will not stage a vote on the House floor to officially launch an impeachment inquiry.