WASHINGTON (AFP) - Mr Donald Trump, clearly angered by news reports that he has grown depressed and sullen over his fading presidential prospects, has issued some of his sharpest attacks on the media.

"I am not running against Crooked Hillary Clinton," the Republican presidential candidate said in a speech late on Saturday in Fairfield, Connecticut. "I'm running against the crooked media."

Mr Trump seemed particularly upset with a New York Times article that quotes unnamed associates of his as saying that in private "his mood is often sullen and erratic". Republicans close to his campaign were quoted as saying he was "exhausted, frustrated and still bewildered" by the political process.

The real estate tycoon returned to his message on Sunday, tweeting: "My rallies are not covered properly by the media. They never discuss the real message and never show crowd size or enthusiasm."

Later, amid a flurry of further tweets on the subject, he added: "It is not 'freedom of the press' when newspapers and others are allowed to say and write whatever they want even if it is completely false!"

Mr Trump has complained for months about media coverage. He has stripped a long list of news organisations - including the New York Times, Buzzfeed, Politico and the Washington Post - of their credentials, and vowed that as president, he would make it easier to sue news outlets.

But media monitors say he has received more extensive coverage than any candidate in years.

A prominent American journalist, Mr James Fallows of The Atlantic magazine, suggested Mr Trump's very criticism reflected a degree of desperation, tweeting on Sunday: "I've seen winning campaigns and losing ones. 100% Iron Law: Campaign saying 'our problem is the media' is campaign on way down."

But Mr Paul Manafort, the Trump campaign chairman, pushed back against that notion during an appearance Sunday on CNN, saying: "The campaign is moving forward and very strong. We raised over US$132 million (S$178 million) in the last two months."

He noted that Mr Trump had visited key battleground states such as Pennsylvania, Ohio and Florida repeatedly and was "starting to get traction in those states."

However, recent polls have shown Mr Trump's numbers sagging badly in those battleground states, notably hurt by his critical comments about the Muslim parents of a fallen US soldier, and what some saw as his suggestion that "Second Amendment people" - gun lovers - take their dislike for Mrs Clinton into their own hands.

Mr Manafort repeated the Trump claim that his Second Amendment remark was meant purely as an exhortation to vote.

But even one of Mr Trump's top advisers, Senator Jeff Sessions of Alabama, conceded on Sunday that the candidate needed to communicate "more effectively". "He's got to wrestle in his own heart, how does he communicate who he is, what he believes, the change he thinks he can bring to America," he said on ABC.

"He does need to communicate - and I think he can - more effectively."

The CNN interviewer also asked Mr Manafort about mounting pressure on Mr Trump to release his tax returns after Mrs Clinton released hers on Friday.

The channel broadcast video of Mr Trump urging Mr Mitt Romney, the Republican candidate in 2012, to release his returns at the time, saying: "If you didn't see the tax returns, you would think there is almost, like, something wrong." Mr Manafort repeated Mr Trump's explanation that he is under audit by the Internal Revenue Service.

"When that's completed, he'll release the returns," Mr Manafort said, adding that Mrs Clinton's returns showed income coming from "people who benefited from her State Department term as well", referring to her time as secretary of state.

"I haven't seen stories on that yet."