Hillary Clinton on Thursday blasted her likely general election rival Donald Trump as a threat to US democracy and declared him unqualified to be president, unleashing some of her toughest criticism yet of the Republican presumptive nominee.

"The threat that Donald Trump poses is so dramatic to our country, to our democracy and our economy," the former secretary of state told CNN in an interview.

"I know how hard this job is, and I know that we need steadiness as well as strength and smarts in it," she added. "And I have concluded he is not qualified to be president of the United States."

The statement marks a hardening of her position on Trump.

On May 3, Clinton declined to answer directly when asked by MSNBC whether Trump was qualified to be commander in chief, although Clinton said the New York billionaire "has given no indication he understands the gravity of the responsibilities" that go with the job.

Clinton's criticism Thursday was more biting, and left no doubt that the gloves are off, perhaps for the duration of the contest that culminates with the November 8 election.

She pointed to Trump's recent criticism of top officials of US ally Britain; his stated willingness to meet with North Korea's reclusive dictator; his assessment that NATO is "obsolete"; and his suggestion of opening the door to more countries, such as Japan or South Korea, having nuclear weapons.

Such positions are "dangerous," Clinton said, adding that the totality of Trump's alarming statements since he launched his presidential campaign, including calling for a ban on Muslims entering the United States, amounts to a worrying "pattern."

"I think if you go through many of his irresponsible, reckless, dangerous comments, it's not just somebody saying something off the cuff," Clinton said.

Trump's antagonistic tone toward Muslims would make it far harder for Washington to work with Middle Eastern governments in fighting extremism, she said.

"We have seen how Donald Trump is being used to essentially be a recruiter for more people to join the cause of terrorism," Clinton said.

"When you run for president of the United States, the entire world is listening and watching."


'Insurmountable lead'

Clinton's pugnacity came as she all but declared victory in her nomination battle with Bernie Sanders.

"I will be the nominee for my party. That is already done," she said.

"In fact, there's no way that I won't be."

Sanders, a US senator from Vermont, insists he is still locked in a fierce battle for the nomination and has vowed to take the race all the way to June 7, when California, the most populous state in the union, holds its primary.

But Clinton essentially said it was time for Sanders to accept the inevitability of her winning the nomination.

"I'm three million votes ahead of him and I have an insurmountable lead in pledged delegates," she said.

With only six US states left to vote, Clinton's delegate lead, including so-called super-delegates, is formidable. She has 2,297 delegates compared to Sanders's 1,527, according to a CNN tally. A candidate needs 2,383 delegates to be declared the nominee.

It was perhaps Clinton's most forceful declaration to date that the race is basically over, as she turns her focus on Trump and their general election showdown.

She also expressed confidence that Sanders will eventually offer his support -- as she did for then-senator Barack Obama during their intense 2008 nomination battle -- once the Democratic race comes to a halt.

"I have every confidence we're going to be unified," Clinton said, but Sanders "has to do his part to unify."

The Sanders campaign said Clinton was jumping the gun, pointing to the self-declared democratic socialist's recent wins in Indiana, West Virginia and Oregon.

"And with almost every national and state poll showing Senator Sanders doing much, much better than Secretary Clinton against Donald Trump, it is clear that millions of Americans have growing doubts about the Clinton campaign," Sanders's spokesman Michael Briggs said in a statement.

Trump has yet to respond publicly to Clinton's broadside. But senior adviser Stephen Miller warned that Trump will be turning up the heat during the campaign.

"I think we're definitely going to be a lot more aggressive, on every kind of attack, because Hillary Clinton would be an extraordinarily dangerous person to put in the White House," Miller told CNN.