US Republican presidential candidate Donald Trump has detailed $US5.6 million ($A7.79 million) in contributions he raised for military veterans, while sharply criticising the media for questioning him for months about what happened to the money.
At a combative news conference at Trump Tower in Manhattan on Tuesday, the billionaire accused the media of failing to give him credit for raising the funds at an event in January in Iowa.
"The press should be ashamed of themselves," he told reporters gathered before him.
"You make me look very bad. I've never received such bad publicity for doing a good job."
Trump's tirade may dash the hopes of some Republican leaders who want him to tone down his rhetoric and become more magnanimous now he's the Republican presidential nomination for the election.
Reporters have persistently questioned whether he in fact raised all the money he said he had in January and why it had taken so long to hand donations over to veterans groups.
The Washington Post said Trump only handed over a personal donation of $US1 million last week, four months after announcing it, once the newspaper started asking questions about the money.
Trump said the coverage of his veterans group donations had been close to libellous.
Asked whether he would keep his adversarial stance with reporters if elected president, Trump said: "Yeah, it's going to be like this."
The campaign of Democratic front-runner Hillary Clinton sought to take advantage of Trump's discomfort by holding a conference call with reporters to accuse him of hypocrisy over veterans' issues.
John Douglass, a retired Air Force general and Clinton backer, raised Trump's disparagement a year ago of US Senator John McCain of Arizona, a war veteran.
Trump had said McCain, a former Vietnam prisoner of war, was not a hero because he got captured.
Clinton leads Trump by 11 percentage points in the latest Reuters/Ipsos poll.
According to the May 27-31 survey of likely voters, 46 per cent support the former US secretary of state while 35 per cent back Trump.
California Governor Jerry Brown on Tuesday endorsed Clinton for the Democratic nomination, saying it was the only way to "stop the dangerous candidacy of Donald Trump".
The New York real estate mogul in Tuesday also bristled at the possibility that Republicans opposed to him might run a third-party candidate as an alternative to Trump or the expected Democratic nominee Clinton.
"Let me tell you these people are losers," said Trump, adding that a third-party candidacy would guarantee Clinton wins the White House and deny Republicans the chance to put conservatives on the Supreme Court.
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