US presidential hopefuls Donald Trump and Hillary Clinton clashed Monday on their domestic and international agendas in a wide-ranging first debate filled with personal attacks and acrimony.

The highly anticipated contest covered the two candidates' economic policies, race relations, the war against Islamic State, and US international ties.

Republican Trump appeared confident initially as he discussed his opposition to trade deals, his views on race relations and fighting crime. When the debate shifted to foreign policy, Democrat Clinton took control and seemed to frequently bait Trump despite his repeated attempts to interrupt.

The first of three debates came as the race has narrowed. An opinion poll released Monday showed a dead heat at the national level, with both nominees receiving 46 per cent of likely voters in a Bloomberg survey.

"You have to judge us," Clinton told voters. "Who can shoulder the immense responsibilities of the presidency?"

Each candidate attempted to paint the other as unfit to lead, with Clinton repeating that Trump does not have the temperament and Trump calling that his "strongest asset."

He charged that Clinton did not have the "stamina" to do the job, but she responded by pointing to her strenuous schedule as secretary of state. He conceded her experience, but characterized it as "bad experience."

The discussions hit on a number of controversies from Trump's decision not to release his tax returns to Clinton's use of a private email server while secretary of state and Trump's advocating falsely that President Barack Obama was not born in the United States.

On his decision not to release his tax returns, Trump said he would do so if Clinton releases thousands of deleted emails from the private server.

"This is something the American people deserve to see," Clinton said of Trump's taxes. "There's something he's hiding and we'll continue to guess at what that is."

She also charged he had begun his political career advocating a "racist lie" about Obama's place of birth in an attempt to discredit him.

The first segment covering economic policies saw Trump accuse Clinton of supporting damaging trade agreements.

He took Clinton to task for the North American Free Trade Agreement (NAFTA) with Canada and Mexico signed by her husband president Bill Clinton, citing lost manufacturing jobs in several states that will prove decisive in the November 8 elections.

He charged that as-yet unratified Trans-Pacific Partnership (TPP) with 11 other nations would be "almost as bad."

Clinton, who has been opposed to the TPP as a candidate despite earlier advocating the deal, said she would evaluate agreements based on whether they create US jobs.

The two then sparred on a question about race relations that led to a discussion of criminal justice reform, which Clinton said she supported to restore trust between communities and the police.

But Trump said the policies of Democrats had been disastrous for US inner cities.

People there, he said, are "living in hell because it's so dangerous." He said he favours a policy of "stop and frisk," saying it helped bring the crime rate "way down" in New York City before it was overturned.

When the debate moved to international affairs, the former secretary of state vowed to support US allies, while Trump said the US should not be shouldering the burden of the world's defence needs.

"It is essential that America's word be good," Clinton said.

The US "cannot be the policemen of the world," he said, adding that countries like Germany and Japan should pay their fair share.

She blamed Russia for the recent cyberattack on the Democratic campaign offices, saying it is one of Moscow's "preferred methods of trying to wreak havoc and collect information."

Trump, who has been accused of having close ties with Moscow, said it could have been Russia, but it could also be China.

Trump charged that Clinton and Obama created a vacuum when the US withdrew the majority of its forces from Iraq in 2011 after years of war.

Islamic State "wouldn't have been formed if (more) troops had been left behind," he said.

Clinton countered by saying Trump had supported the invasion of Iraq and said more importantly, the agreement about when US troops would leave Iraq was made by Republican president George W Bush, not Obama.

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