DES MOINES, United States (AFP) - White House hopeful Hillary Clinton called for global unity to crush the Islamic State in Iraq and Syria (ISIS), as the carnage in Paris took centre stage at Saturday's (Nov 14) Democratic presidential debate.

The three candidates began their debate with a moment of silence for the victims in France, bringing Friday's horrific attacks an ocean away to the forefront of the White House race as they dominated the first half hour of the political showdown.

Clinton, liberal US Senator Bernie Sanders and former Maryland governor Martin O'Malley united in calling for the destruction of the extremists accused of massacring at least 129 people in the French capital.

"We are not at war with Islam," said the former secretary of state, choosing her words with care as she warned ordinary Muslims should not be viewed as a threat. "We are at war with violent extremism."

"Our prayers are with the people of France tonight, but that is not enough," she said. "We need to have a resolve that will bring the world together to root out the kind of radical jihadist ideology that motivates organisations like ISIS, a barbaric, ruthless, violent jihadist terrorist group."

ISIS claimed responsibility for the coordinated attacks on a Paris concert hall, restaurants and bars, and outside France's national stadium - calling it retribution for French air strikes in Syria.

"It cannot be contained, it must be defeated," Clinton said of the group which has overrun swathes of Syria and Iraq.

While they displayed equal determination to eradicate extremism, fissures appeared between the candidates on whether the United States should lead the struggle.

Clinton said American leadership was critical in the effort, with all the diplomatic tools at Washington's disposal beyond just military might, "but this cannot be an American fight."

That drew a sharp disagreement from O'Malley. "This actually is America's fight," he insisted. "America is best when we are actually standing up to evil in this world."

Relatively hawkish Clinton, self-described democratic socialist Sanders and low-polling O'Malley took the stage in Des Moines, Iowa for their second Democratic showdown in the 2016 primary cycle.

With 79 days before the first state-wide vote in Iowa, frontrunner Clinton has reinforced her status as the woman to beat in the race. Her poll numbers, which suffered a hit between spring and late summer when voters questioned her handling of an email scandal, have risen steadily since mid September, to more than 54 per cent today according to a RealClearPolitics average.

Sanders's support has risen steadily to 33 per cent, while O'Malley is languishing at less than 3.0 per cent in the average.

With Sanders eager to take the fight to Clinton on the issue of the economy - he is calling for an economic revolution, while knocking Clinton for her ties to Wall Street - the refocus on terrorism shifted the early portion of the debate clearly in favour of the former top diplomat, fluent in foreign policy.

But Sanders stood his ground, arguing that the Iraq war, which then-senator Clinton voted to authorise in 2002, laid the foundation for the surging extremist threat that once more sowed carnage on Friday.

"I would argue that the disastrous invasion of Iraq, something that I strongly opposed, has unravelled the region completely and led to the rise of Al-Qaeda and to ISIS," Sanders said.

The Iraq war, he repeated, "was one of the worst foreign policy blunders in the modern history of the United States."

On the economic front, the candidates sparred - gently, by comparison with their Republican rivals who have already clashed in four on-stage debates - over how to increase wages, expand the work force and rein in big banks.

Clinton advocated raising the minimum wage to US$12 (S$17) an hour, while Sanders pressed for US$15, double that of today.

And while Clinton has expressed support for a fee on large banks to curb certain risk-taking, as well as for taxes on certain high-frequency trades, Sanders demanded far more dramatic steps against Wall Street.

"I will break up these banks" if elected, Sanders said.