Democratic presidential candidate Hillary Clinton on Thursday held her first formal press conference in nearly nine months, going on the offensive against Republican nominee Donald Trump.

The former US secretary of state had been under increasing criticism for failing to make herself available to the press, despite daily interviews, public forums and a recent chat with reporters on her campaign jet.

The two major-party nominees each appeared Wednesday night in a television forum focused on defence issues, in which Trump said he would have "a wonderful relationship" with Russian President Vladimir Putin.

"I think I'll be able to get along with him," Trump said. "He's been a leader. Far more than our president has been a leader."

Clinton blasted Trump on Thursday from a lectern on the tarmac of an airport in White Plains, New York.

"Last night was yet another test, and Donald Trump failed yet again," she said in a nearly 17-minute press conference. "We saw more evidence that he is temperamentally unfit and totally unqualified to be commander in chief."

Trump had described US generals as humiliated and marginalized under US President Barack Obama, and said as president he would have "different generals."

Clinton accused Trump of having "trash talked" US military leaders.

"That's how he talks about distinguished men and women who have spent their lives serving our country, sacrificing for us. That's how he would act as commander in chief," she said.

"Meanwhile, bizarrely, once again he praised Russia's strongman, Vladimir Putin, even taking the astonishing step of suggesting that he prefers the Russian president to our American president ... It suggests he will let Putin do whatever Putin wants to do, and then make excuses for him."

Clinton was asked in the press conference about the relatively close polling despite Trump having antagonized so many voters.

She had opened a wide lead in voter surveys last month as Trump made a series of inflammatory statements. More recently, however, her edge seemed to narrow as further revelations about her use of a private email server at the State Department revived the long-running scandal.

The average of the latest nationwide surveys, compiled on the website Real Clear Politics, shows Clinton leading Trump by less than 3 percentage points.

"I've always thought this was going to be a close election," she said.

"I feel we're in a strong position."

Later Thursday, Trump raised the email issue again.

"She's got to get her act together. Remember, Hillary Clinton was emailing about the drone programme, among many other extremely sensitive matters. This is yet more evidence that Clinton is unfit to be your commander in chief," he said in Cleveland, Ohio.

Trump accused Clinton of refusing "to take accountability for her failed policies in the Middle East that have produced millions of refugees, unleashed horror of radical Islamic terrorism all over, and made us less safe than ever before."

As secretary of state from 2009-13, Clinton was part of Obama's national security team when US special forces killed al-Qaeda terrorist network leader Osama bin Laden in a 2011 raid in Pakistan.

"We should make it a top priority to hunt down the leader of [the Islamic State movement], Abu Bakr al-Baghdadi, and bring him to justice, just as we did with Osama bin Laden," she said.

"As with that operation, getting al-Baghdadi will require a focused effort driven at the highest levels, but I believe it will send a resounding message that nobody directs or inspires attacks against the United States and gets away with it."

Clinton, who has struggled to get attention for her policy speeches amid media focus on her opponent's every utterance, hammered hard at Trump, a New York real estate tycoon with no previous political experience.

"We're on the brink of making a very critical decision for our country," she said. "And if I were not the candidate, if I were not the nominee, if I were just a concerned citizen, I'd be out here doing everything I could to sound the alarm about someone like Donald Trump getting anywhere near the White House."

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