Clinton policy on Syria would lead to WWIII

US Republican presidential nominee Donald Trump said on Tuesday that Democrat Hillary Clinton's plan for Syria would "lead to World War Three," because of the potential for conflict with military forces from nuclear-armed Russia.

Meanwhile, Democratic presidential candidate Hillary Clinton warned her supporters on Tuesday against complacency as opinion polls show her holding a clear lead over Republican rival Donald Trump with exactly two weeks left until the Nov 8 election.

Clinton received a further boost when Colin Powell, who served as Republican President George W Bush's secretary of state, said he would vote for her, according to media reports.

"I hope you will come out and vote because it's going to be a close election. Pay no attention to the polls. Don't forget, don't get complacent, because we've got to turn people out," she said in Florida, one of the key swing states that will decide the outcome of the election.

In an interview focused largely on foreign policy, Trump said defeating Islamic State is a higher priority than persuading Syrian President Bashar al-Assad to step down, playing down a long-held goal of US policy.

Trump questioned how Clinton would negotiate with Russian President Vladimir Putin after demonizing him; blamed President Barack Obama for a downturn in US relations with the Philippines under its new president, Rodrigo Duterte; bemoaned a lack of Republican unity behind his candidacy, and said he would easily win the election if the party leaders would support him.

On Syria's civil war, Trump said Clinton could drag the United States into a world war with a more aggressive posture toward resolving the conflict.

Clinton has called for the establishment of a no-fly zone and "safe zones" on the ground to protect non-combatants. Some analysts fear that protecting those zones could bring the United States into direct conflict with Russian fighter jets.

"What we should do is focus on ISIS. We should not be focusing on Syria," said Trump as he dined on fried eggs and sausage at his Trump National Doral golf resort. "You're going to end up in World War Three over Syria if we listen to Hillary Clinton.

Clinton's campaign dismissed the criticism, noting that both Republican and Democratic national security experts have denounced Trump as unfit to be commander-in-chief.