Donald Trump has suggested that Russia's President Vladimir Putin had been a better leader than US President Barack Obama.

The Republican presidential candidate's remarks came at a televised forum on Wednesday in New York City that paired him with his Democratic rival Hillary Clinton in separate appearances.

He suggested that US generals had been blocked by the policies of Obama and Clinton, who served as Obama's first secretary of state.

Trump's praise for Putin and his suggestion that the US and Russia form an alliance to defeat the Islamic State of Iraq and the Levant (ISIL) group could trouble foreign policy experts who feel Russia is interfering with efforts to end the Syrian civil war.

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"If he says great things about me, I'm going to say great things about him," Trump said of Putin at NBC's Commander-in-Chief forum attended by military veterans.

"Certainly in that system, he's been a leader, far more than our president has been."

It was the first time Trump, 70, and Clinton, 68, had squared off on the same stage since accepting their respective parties' presidential nominations in July for the November 8 election.

Classified information

Clinton was questioned over her handling of classified information while using a private email server during her tenure at the state department.

Matt Lauer, the moderator, pressed her about her handling of emails from a private server while secretary of state from 2009 to 2013.

The issue has raised questions about whether she can be trusted to serve as president.

Asked if he would be prepared on Day One to be commander in chief, Trump said: "One hundred percent." [Mike Segar/Reuters]

James Comey, FBI director, had declared her "extremely careless" in her handling of sensitive material but did not recommend charges against her.

"I did exactly what I should have done and I take it very seriously, always have, always will," she said.

Clinton said none of the emails she sent or received was marked top secret, secret or classified, the usual way such material is identified.

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The event offered a prelude to how Clinton and Trump will deal with questions on national security issues in their three upcoming presidential debates later in September and in October.

Clinton began the forum saying her long experience in government as a US senator and secretary of state made her uniquely qualified to serve as president.

Trump faced questions about his fitness for office. Asked if he would be prepared on Day One to be commander in chief, Trump said: "One hundred percent."

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Source: Agencies