Chief strategist Steve Bannon's days in the White House are over.

Bannon, 63, had been a key adviser to U.S. President Donald Trump's general election campaign and a forceful but contentious presence in a divided White House.

"White House Chief of Staff John Kelly and Steve Bannon have mutually agreed today would be Steve's last day," White House spokesperson Sarah Sanders said in a statement. "We are grateful for his service and wish him the best."

The statement came out shortly after The Associated Press and New York Times reported, citing anonymous sources, that his departure was imminent.

It wasn't immediately clear if Bannon resigned or was fired. The New York Times had reported that Bannon gave a letter of resignation earlier this month, but numerous media reports state he was fired or pushed out.

Citing an anonymous source, the Washington Post reported Bannon's departure was solely Kelly's decision.

The former leader of conservative Breitbart News has pushed Trump to follow through with his campaign promises. But he's also sparred with some of Trump's closest advisers, sources have told The Associated Press, including son-in-law Jared Kushner.

Bannon, left, and Trump's son-in-law, Jared Kushner, right, reportedly had clashes before Bannon's departure Friday. (Kevin Lamarque/Reuters)

When Kelly took over from Reince Preibus as chief of staff, he was said to be undertaking a review of administration personnel, leaving Bannon's future in doubt.

The president refused to give a vote of confidence during his freewheeling news conference Tuesday.

"He's a good person. He actually gets very unfair press in that regard," Trump said. "But we'll see what happens with Mr. Bannon."

Bannon was viewed by many as Trump's connection to his base of most-committed voters and the protector of the disruptive, conservative agenda that propelled the celebrity businessman to the White House.

"It's a tough pill to swallow if Steve is gone, because you have a Republican West Wing that's filled with generals and Democrats," said former campaign strategist Sam Nunberg, shortly before the news of Bannon's departure broke. "It would feel like the twilight zone."

Breitbart News says Bannon has returned to the website, back as its executive chairman. Bannon left Breitbart just a little over a year ago to join Trump's presidential campaign.

From Breitbart, there was a dramatic one-word warning.

"#WAR," tweeted Joel B. Pollak, a senior editor at large at the news site.

In his first public remarks after his exit, Bannon said he still backed Trump.

"I'm leaving the White House and going to war for Trump against his opponents, on Capitol Hill, in the media and in corporate America," Bannon told Bloomberg News.

Critical interview

According to the Times, Bannon offered his resignation on Aug. 7. He sounded off earlier this week in an interview with American Prospect magazine, stating that there was no military solution in North Korea, in contrast to declarations of Trump and Defence Secretary James Mattis.

2/ ... Bannon put in resignation on Aug 7 to be effective Aug 14, to one year anniversary of him joining Trump. —@maggieNYT

He also said the U.S. is losing an economic war with China, an issue not getting enough attention in the administration. "If we continue to lose it, we're five years away, I think, 10 years at the most, of hitting an inflection point from which we'll never be able to recover."

After serving in the navy, Bannon earned an MBA at Harvard and worked in investment banking. He helped back movies and television shows, his involvement in the sale of Castle Rock Entertainment earning him a stake in the syndication of Seinfeld and other shows.

After the Sept. 11 attacks, Bannon wrote and produced documentaries based on his belief the West was engaged in a clash of civilizations with radical Islam.

Bannon is shown with then White House chief of staff Reince Priebus on March 17. Both are among a number of high-profile departures in the first seven months of the Trump administration. (Jim Bourg/Reuters)

While hosting Trump on his Sirius XM radio show in 2015, he also lamented the presence of foreign-born tech professionals in California.

"When two-thirds or three-quarters of the CEOs in Silicon Valley are from South Asia or from Asia, I think … a country is more than an economy. We're a civic society," said Bannon.

Democratic House minority leader Nancy Pelosi welcomed the move, but said that Trump's "repulsive policies" are the biggest problem in the White House.

Steve Bannon's exit does not erase <a href="https://twitter.com/realDonaldTrump">@realDonaldTrump</a>’s long record of lifting up racist viewpoints & advancing repulsive policies. <a href="https://twitter.com/hashtag/BannonOut?src=hash">#BannonOut</a> —@NancyPelosi

Glad to see Bannon gone; he never belonged in the WH. But the problem persists, since the most profound source of division cannot be fired. —@RepAdamSchiff

Bannon becomes the latest departure amid a number of resignations and firings in the first seven months of the Trump administration. They include: