WASHINGTON - US President Donald Trump has attacked CNN's president Jeff Zucker on Twitter, saying that he should be fired.

President Trump said CNN's "hatred and extreme bias" towards him made the media organisation "unable to function".

"Little Jeff Z has done a terrible job, his ratings suck, & AT&T should fire him to save credibility!" he tweeted.

Mr Zucker is currently on six weeks' leave from CNN to recover from having heart surgery. AT&T said it had no comment.  CNN declined to comment on the latest tweets, but directed the BBC to its response on Twitter to President Trump's tweets on Wednesday. The broadcaster also pointed out that its ratings this month - 707,000 viewers - were its second highest ever achieved for the month of August.

Further, CNN highlighted the fact that comScore's Multi-Platform Media Metrix ranked CNN as number one in multiplatform visitors, mobile visitors, video starts, millennial reach and social following.

President Trump has long complained about CNN, maintaining that it is dishonest and focused on its own agenda.

CNN reporter Kaitlan Collins was barred from a White House event in July after asking questions about Russian President Vladimir Putin and Mr Trump's ex-lawyer, Michael Cohen.

In a second tweet on Thursday, President Trump also criticised NBC News.

"What's going on at CNN is happening, to different degrees, at other networks - with NBC News being the worst," he said.

He added that it was "good news" that NBC News chairman Andy Lack could be facing dismissal over his handling of a series of sexual harassment allegations against high-profile news presenters, as well as the hiring of former Fox News anchor Megyn Kelly.

Over the last few months, President Trump has also targeted the Washington Post, which is owned by Amazon founder Jeff Bezos.

President takes aim at Watergate scandal reporter

President Trump took aim at Carl Bernstein, one of the reporters who helped bring down Richard Nixon's presidency, accusing him of making up stories. Bernstein and Bob Woodward led the Washington Post team that investigated the 1972 break-in at the Democratic National Committee HQs at the Watergate complex, which eventually led back to the W House, prompting a scandal that forced Nixon to resign in 1974. "Sloppy @carlbernstein, a man who lives in the past and thinks like a degenerate fool, making up story after story, is being laughed at all over the country!" Trump tweeted.

He also slammed CNN - a frequent target of criticism from the president - saying that the network "is being torn apart from within based on their being caught in a major lie and refusing to admit the mistake." Bernstein was one of the reporters on a CNN story that quoted sources as saying Trump's former attorney Michael Cohen claimed the US president knew in advance about a June 2016 meeting in which Russians were expected to share dirt on then-election rival Hillary Clinton. The story has since been called into question as one of the sources - Cohen's own lawyer Lanny Davis - recanted, but the network has stood by it.

The network hit back, replying to Trump's tweet.

"Make no mistake, Mr. President, CNN does not lie. We report the news. And we report when people in power tell lies. CNN stands by our reporting and our reporters. There may be many fools in this story but @carlbernstein is not one of them," it said. Bernstein also pushed back against the president.

"I have spent my life as a journalist bringing the truth to light, through administrations of both parties. No taunt will diminish my commitment to that mission, which is the essential role of a free press," he wrote on Twitter.

"@CNN stands by its story, and I stand by my reporting," Bernstein added.

Trump's attack on Bernstein comes as the White House braces for the release of a book by Woodward.

"Fear: Trump in the White House," which comes out on September 11, "reveals in unprecedented detail the harrowing life inside Donald Trump's White House and how the president makes decisions on major foreign and domestic policies," publisher Simon & Schuster said.

The network hit back, replying to Trump's tweet. "Make no mistake, Mr. President, CNN does not lie. We report the news. And we report when people in power tell lies. CNN stands by our reporting and our reporters. There may be many fools in this story but @carlbernstein is not one of them"